INDUSTRIAL CORROSION AND
CORROSION CONTROL TECHNOLOGY
H.M. Shalaby
A. Al-Hashem
M. Lowther
J. Al-Besharah
(Editors)
Published By
INDUSTRIAL CORROSION AND CORROSION CONTROL
TECHNOLOGY
Sponsored by the Kuwait Institute for Scientific Research (KISR), the Kuwait
Foundation for the Advancement of Science (KFAS), the Kuwait National
Petroleum Company (KNPC), the Kuwait Oil Company (KOC), Ministry of
Electricity and Water (MEW), Kuwait University (KU), Ministry of Oil (MO),
the Gulf Cooperation Council-General Secretariat (GCC), Kuwait Chemical
Society (KCS), Organization of Arab Petroleum Exporting Countries (OAPEC),
and Petrochemical Industries Company (PIC).
INDUSTRIAL CORROSION AND CORROSION
CONTROL TECHNOLOY
Proceedings of the 2nd Arabian Corrosion Conference
Kuwait, October 12-15, 1996
Editors
H.M. Shalaby, A. Al-Hashem, M. Lowther and J. Al-Besharah
PUBLISHED BY KUWAIT INSTITUTE FOR SCIENTIFIC RESEARCH P.O. BOX 24885, 13109 SAFAT, KUWAIT
Published by
Kuwait Institute for Scientific Research P.O. Box 24885, 13109 Safat, Kuwait Publication Number: KISR 4890
Copyright ® 1996 by Kuwait Institute for Scientific Research
The papers were reviewed for their technical contents. Editing was restricted to matters of format, general organization and retyping. The editors assume no responsibility for the accuracy, completeness or usefulness of the information disclosed in this book. Unauthorized use might infringe on privately owned patents of publication right. Please contact the individual authors for permission to reprint or otherwise use information from their papers This book was printed in Kuwait
The 2nd Arabian Corrosion Conference was held in the state of Kuwait during the period 12-15 October, 1996 under the auspices of H.H. Sheikh Saad Al-Abdullah Al-Salem Al-Sabah, Kuwait’s Crown Prince and Prime Minister. The present conference was scheduled to be held in Kuwait during 27-30 April, 1991, however, it was postponed due to the events that encompassed Kuwait and the Gulf region in 1990-1991.
The 1st Arabian Corrosion Conference was held in Kuwait during 4-8 February, 1984. It was attended by over 300 scientists and engineers, representing 26 countries. The conference proceedings were published in two volumes by Pergamon Press under the title “Corrosion:
Industrial Problems, Treatment and Control Techniques”. The conference provided a forum
for the exchange of ideas between scientists and engineers from the region with their counterparts from the industrialized countries.
The patronage of the present conference, the organizing bodies, and the emphasis on industrial corrosion and corrosion prevention reflect the keen interest of the countries in the region in actively combating corrosion problems. This also reflect the recognition of the economic impact resulting from the corrosion of materials.
Kuwait and the other Arab countries rely heavily on the utilization of metallic materials in their oil-based industries. Seawater derived from the Arabian Gulf is used in water desalination and as an industrial cooling media. The salinity of the Arabian Gulf seawater is very high when compared to other seawater bodies. The Arabian Gulf countries are located in an arid environmental zone where the temperature during the summer months could reach 50oC and the humidity during the autumn season could become 80% in some of the Gulf states. All these factors contribute to the enhancement of the rate of corrosion of metals and/or cause unpredictable service failures.
The program of the present conference includes a field visit to one of Kuwait’s modern refineries and a trip to one of Kuwait’s oil fields. The success of the conference is perhaps difficult to assess. However, the quality of the papers in this volume provides some indication.
PREFACE
The technical program of the present conference includes five plenary lectures and fifty three scientific presentations from about twenty two countries. A number of honorary speakers, carefully selected from high ranking officials and policy makers, were also invited to address the conference. The honorary speakers are expected to provide an overview of the magnitude of corrosion related problems in the Middle East as well as the avenues of linkage between corrosion science and industrial applications. The conference papers were carefully selected to include a blend of fundamental and applied research, and industrial experience. Such a blend was thought to be essential for providing the participants from both industry and academia with a chance to become familiar with the challenges facing each group and the preventive actions to meet them. The papers were refereed in terms of scientific and technical content and format in accordance with internationally accepted standards.
The papers in the proceedings are grouped in the following sections for quick reference:
•
Plenary Lectures•
Oil Field Corrosion•
Corrosion in Refinery and Petrochemical Industries•
Seawater Corrosion•
Corrosion in the Building Industry•
Fundamental Aspects•
Corrosion Protection and Monitoring•
Corrosion Management•
Novel TechniquesThe plenary papers are mostly reviews covering important topic related to the objectives of the conference. The remaining papers cover various topics of major importance to corrosion in general and particularly to the oil-based and desalination industries. A good number of papers delt with corrosion protection and new techniques for corrosion monitoring.
The task of editing this volume was facilitated by the efforts of the International Advisory Committee and the Scientific Committee for the conference who reviewed all the papers. The editorial board gratefully acknowledge these efforts; the cooperation, time and effort of all authors ; and the management of the Kuwait Institute for Scientific Research for allocating the required resources to prepare the manuscript of this volume.
The Editors
Foreword...v Preface...vi Organizing Committees...xi Acknowledgement...xii PLENARY LECTURES Corrosion Management V. Ashworth...1
The Deterministic Prediction of Damage D.D. Macdonald...17
Relevance of Laboratory Corrosion Tests in Corrosivity Assessment and Materials Selection: Case Studies R.D. Kane...37
Corrosion of Condensers in Multi Stage Flash Evaporation Distillers A.M. Shams El Din...49
Correct Materials Selection for Desalination -The Key To Plant Reliability J.W. Oldfield...67
OIL FIELD CORROSION Corrosivity Prediction for Co2/H2s Production Environments S. Srinivasan and R.D. Kane...89
Testing of Drilling Fluids Formulated From Tabuk Formation Clays M.N.J. Al-Awad, A.S. Dahab and M.E. El-Dahshan...111
Preventing Sulfate Scale Deposition in Oil Production Facilities C.J. Hinrichsen, M.J. McKinzie, S. He, J. Oddo, A.J. Gerbino, A.T. Kan, and M.B. Tomson...127
Concerns Over the Selection of Biocides for Oil Fields and Power Plants: A Laboratory Corrosion Assessment J. Alhajji and M. Valliappan...135
Evaluation of Microbially Influenced Corrosion Risks and Control Strategies in Seawater and Produced Water Injection Systems, Kuwait P.F. Sanders, M. Salmanand K. Al-Muhanna...149
Hydrogen Degradation of Steel - Diffusion and Deterioration M. Farzam...165
Control Strategies for Thermophilic Sulphate-Reducing Bacteria P.F. Sanders, H.M. Lappin-Scottand C.J. Bass...179
Corrosion Evaluation of Austenitic and Duplex Stainless Steels in Simulated Hydrogen Sulphide Containing Petrochemical Environments K. Saarinen and E. Hamalainen...191
Damage of Pump Linkages and Tool Joints Caused by Crack Corrosion
A. Kinzel...201
Analysis of Soils Possibility to Give Rise to Pipe Metal Stress Corrosion Cracking
V.G. Antonov and S.A. Loubenski...209
A Mysterious Downhole Corrosion Failure in an Oil Well
A. Husain and A. Hasan...215 CORROSION IN REFINERY AND PETROCHEMICAL INDUSTRIES
Methodologies for Assessment of Crude Oil Corrosivity in Petroleum Refining
S. Tebbal and R.D. Kane...225
New Nickel Alloys Solve Corrosion Problems of Various Industries
D.C. Agarwal and W.R. Herda...233
Macro-Micro Segregation Bands (MMB) as a Main Factor Influencing Steel Applicability for the Petroleum Industry
A. Mazur...245
Fluid Catalytic Cracking Interstage and High-Pressure Cooler Corrosion
S.M. Halawani...255
Assessment of Cracks in a High Pressure Multilayered Reactor for its Fitness for Purpose
A.M. Askari, M.I. AL-Kandari and P.K. Mukhopadhyay...263
Polythionic Acid Stress Corrosion Cracking of Incoloy 800: Case Study and Failure Analysis
M.S. Mostafa and S.A. Hajaj...273
Corrosion of Tube Heaters in Refineries: Symptoms and Cures
A. Attou , A. Rais and H. Smamen...283 SEAWATER CORROSION
Super Duplex Grade UNS S32750 for Seawater Cooled Heat Exchangers
P.A. Olsson and M.B. Newman...289
Evaluation of Aluminum Alloy 5083 Weldments to Stress Corrosion Cracking in Seawater
A. Saatchi, M.A. Golozar and R. Mozafarinia...301
Cavitation Corrosion Behavior of Some Cast Alloys in Seawater
A. Al-Hashem, P.G. Caceres and H.M. Shalaby...311
Microbiologically Induced Corrosion of a Stainless Steel Pipe
H.H. Lee, M. Ali and K. Al-Omrani...323
A Laboratory Study of Service Failure of Al-Brass Tubes in Arabian Gulf Seawater
H.M. Shalaby, W.T. Riad and V.K. Gouda...329 CORROSION IN THE BUILDING INDUSTRY
Corrosion of Reinforced Concrete Structures and the Effects of the Service Environment
S. Al-Bahar and E.K. Attiogbe...341
Corrosion of Concrete in Seawater
The Effect of the Type of Copper on its Corrosion Behavior in Kuwait’s Soft Tap Water
H.M. Shalaby and F.M. Al-Kharafi...371 FUNDAMENTAL ASPECTS
Corrosion Behavior of Vanadium in Aqueous Solutions
W.A. Badawy, F.M. AI-Kharafi and M.H. Fath-Allah...383
The Effect of UV Irradiation on Passive Films Formed on Type 304 and 316 Stainless Steels
M.S. Al-Rifaie, C.B. Breslin, D.D. Macdonald and E. Sikora...395
Kinetics of High Temperature Corrosion of a Low Cr-Mo Steel in Aqueous NaCl Solution
W.A. Ghanem, F.M. Bayyoum and B.G. Ateya...407
Corrosion and Passivation Behaviour of Aluminium and Aluminium Alloys: Mechanism of the Corrosion Process
F.M. AI-Kharafi, W.A. Badawy and A.S. El-Azab...417
The Susceptibility of Molybednum and Vanadium-Bearing Austenitic Stainless Steel Weldments to Intergranular Corrosion
M.K. Karfoul...431
Effect of Crystallization on the Corrosion Behavior of Amorphous FeCr9P6C3Si0.2 Alloy in 1 M H2SO4
F. Hajji, S. Kertit, J. Aride and M. Ferhat...441 CORROSION PROTECTION AND MONITORING
Experience With VOC-Compliant Waterborne and High Solids Coatings in Corrosive Environments
P Kronborg Nielsen...449
Anticorrosive Film-Forming Nonpolluting Products Achieved in Romania
R. Serban, N. Moga and E. Stockel...461
Cathodic Protection Under Disbonded Coatings of 56 Inch Gas Pipeline Along the Kangan-Shiraz
M. Pakshir...471
Synergistic Effect Existing Between and Among a Phosphonate, Zn2+, and Molybdate on the Inhibition of Corrosion of Mild Steel in a Neutral Aqueous Environment
S. Rajendran, B.V. Apparao and N. Palaniswamy...483
Evaluation of Corrosion Inhibitors for Carbon Steel, Monel 400 and Stainless Steel 321 in a Monoethanolamine Environment Under Stagnant and Hydrodynamic Conditions
J. Carew, H. Al-Sumait, A. Abdullah and A. Al-Hashem...493
Laboratory Evaluation of the Effects of Ozone on Corrosion Rates and Pitting of Engineering Alloys
S. Nasrazadani...501
A Critical Comparison of Corrosion Monitoring Techniques Used in Industrial Applications
Detection, Localization and Monitoring of Stress Corrosion Cracking, Hydrogen Embrittlement and Corrosion Fatigue Cracks During Service Conditions Using Acoustic Emission
L. Giuliani...521
Electrochemical Monitoring of Aerobic Bacteria and Automation of Biocide Treatments
L. Giuliani...533
Corrosion Monitoring for Integrity of Pipeline
G.L. Rajani...543
Power and Desalination Plants: Pumps, Corrosion and Maintenance
H. Hosni, N.J. Paul and A. Masri...555 CORROSION MANAGEMENT
Impact of Metallic Corrosion on the Kuwait Economy Before and After the Iraqi Invasion: A Case Study
F. Al-Matrouk, A. Al-Hashem, F.M. AL-Kharafi and M. EL-Khafif...567
Corrosion Problems in a Steam Condensate System and Treatment of Condensate for Recovery
G.L. Rajani...581
Improved Cathodic Protection of Above Ground Storage Tank Bottoms: MAA Refinery Experience
A.K. Jain, L. Cheruvu and M.E. Al-Ramadhan...597
Impact on Ship Strength of Structural Degradation Due to Corrosion
M.A. Shama...615 NOVEL TECHNIQUES
Contact Electric Resistance (CER) Technique for Monitoring of Process Plants and for Solving Practical Corrosion Problems
K. Saarinen and T. Saario...627
Design of Radio Frequency Methods for Corrosion Processes Monitoring
Yu.N. Pchel’nikov, Z.T. Galiullin and A.S. Sovlukov...637
A New, Rapid Corrosion Rate Measurement Technique for All Process Environments
A.F. Denzine and M.S. Reading...647
Assessing Corrosion of Thick Marine Paints by Surface Corrosion Potential Mapping (SCM) and AC Impedance Spectroscopy (EIS)
A. Husain...657
Optics and Lasers in Corrosion Laboratory
K. Habib and F. Al-Sabti...669
Author Index...677 Subject Index...679
Jasem Al-Besharah Chairman KISR
Khaled Al-Muhailan Rapporteur KFAS
Abdulhameed Al Hashem Coordinator KISR
Hamdy M. Shalaby Member KISR
Abbas Ali Khan Member KFAS
Hussain Shareb Member OAPEC
Jamal Al-Hajji Member KU
Khaled Shehab Member KNPC
Khalifa Al-Feraij Member MEW
Abdel Monem Bedair Member PIC
Mohammad Ashkanani Member KOC
Mohammad Al-Rasheed Member GCC
Mohammed Al-Qalaf Member KCS
Abdul Khaliq Mustafa Member KISR
Khawla Al-Rifaee Member MO
INTERNATIONAL ADVISORY COMMITTEE
Ahmed M. Shams El Din Member UAE
John Oldfield Member UK
Russel D. Kane Member USA
Digby D. MacDonald Member USA
SCIENTIFIC COMMITTEE
Hamdy M. Shalaby Chairman KISR
Abdulhameed Al Hashem Rapporteur KISR
Khalid Habib Member KISR
Adel Hussein Member KISR
Waheed Badawi Member KU
Afkar Hussain Member KOC
Emad Al Naser Member KOC
Eman A. Razzak Al-Shayji Member KOC
Lakshmipati Cheruvu Member KNPC
ACKNOWLEDGEMENT
The Organizing Committee was deeply honored by the patronage of H. H. The Crown Prince and Prime Minister Sheikh Saad Al-Abdullah Al-Salem Al-Sabah, which reflects his keen interest in science and technology.
The Committee was also grateful for the financial support of the Kuwait Institute for Scientific Research, Kuwait Foundation for the Advancement of Science, Kuwait National Petroleum Company, Kuwait Oil Company, Ministry of Electricity and Water, Kuwait University, Ministry of Oil, the Gulf Cooperation Council, Kuwait Chemical Society, Organization of Arab Petroleum Exporting Countries, and Petrochemical Industries Company.
The Committee would also like to extend its deep appreciation for the effort and time put forth by the distinguished honorary speakers, the members of the International Advisory Committee, and the Scientific Committee.
We would like to thank our colleagues, the members of the working committees, at the Kuwait Institute for Scientific Research and the chairmen and cochairmen of the sessions, who provided unlimited assistance at times when it was really needed. Finally, we feel deeply indebted to the authors of papers and participants for their valuable contribution to the success of the conference
Jasem Al-Besharah
Chairman, Organizing Committee
CORROSION MANAGEMENT
V. Ashworth Global Corrosion
The White House, Victoria Road, Shifnal, England, TF11 8AF
ABSTRACT
The consequences of corrosion are often very costly. Little surprise, therefore, that a substantial engineering effort is directed towards its prevention and control. By contrast, little consideration seems to be directed towards making anti-corrosion effort cost-effective. This paper addresses the problem of ensuring value-for-money corrosion engineering and the possible limitation of unnecessary corrosion control activities.
Corrosion in itself is not important, but the consequences of corrosion failure may well be. So the first step in corrosion management is a corrosion risk assessment to evaluate the risk associated with failure in any item. This is not an evaluation of the risk of failure alone, but of the consequences should that failure occur.
Given an assessment of risk, a strategy of corrosion management can be constructed. This might involve lifetime corrosion control for items identified as producing a high risk. A less rigorous, but monitored, level of protection might be adopted for medium risk items, whilst no action at all may be considered necessary in the case of low risk items. Thus, resources are distributed according to the risk.
Once a strategic approach has been defined, the tactics of corrosion management may be determined. These will include not only the specific corrosion control activity or activities that will be used in any given case, but also any monitoring and inspection requirements that are necessary. The object is to maintain corrosion within acceptable limits at minimum cost in all parts of the facility and throughout the facilitiy’s life.
Key Words: Risk, probability, monitoring, inspection, corrosion management
INTRODUCTION
The purpose of industry is to make a profit from the production of supplies and artefacts. In an increasingly competitive world, there is continuing pressure on prices. If the selling price is under pressure, profitability may only be maintained or increased by cutting costs. Any factor that serves to increase costs represents a tax on profits.
Corrosion is one certain consequence of using engineering materials. Commonly, corrosion will be modest, but not always. In the hydrocarbon production and processing industries and the chemical industry, for example, the exception almost becomes the rule. Since corrosion brings a cost, it impacts profits.
Plenary Lectures
THE COST OF CORROSION
The first formal attempt to assess the cost of corrosion to a nation was made in the UK in 1970 [1]. Since that time, similar studies have been published, in Australia [2], the US [3] and elsewhere. One surprising outcome is that the cost of corrosion to an industrialized nation is relatively constant at approximately 3.5% of the gross domestic product (GDP). To put the matter in context, this is substantially higher than the cost of fires which in the UK is put at ~0.5% GDP.
Sedriks [4] reported the experience of the Dupont Company in the period 1968-71. After examining 685 plant failures, it was concluded that 55% were due to corrosion and 45% to mechanical failure. This may be regarded as a remarkable outcome given that, by the standards of the time, Dupont was corrosion aware and the greater proportion of the material that failed was stainless steel.
The Dupont experience was mirrored by that of Britoil in the UK during the period 1978-88 [5]. As Table 1 shows, 33% of the failures that were analysed were attributed to corrosion.
Table 1. Analysis of Oilfield Failures [5]
Type Frequency (%)
Corrosion (all forms) 33
Fatigue 18
Mechanical damage/overload 14
Brittle fracture 9
Fabrication defects (not welding) 9
Welding defects 7
Other 10 Not infrequently, corrosion hits the headlines because some particularly dramatic failure
occurs resulting in the loss of life and property. At Flixborough Works in the UK, a chemical explosion related to a corrosion failure resulted in 28 fatalities, 36 serious injuries, virtual destruction of the plant and damage to some 2000 third party properties [6]. In Guadalajara, in the early 1990's, stray current corrosion of a water pipe produced a failure that caused erosion-corrosion of an adjacent gasoline line [7]. The leaking gasoline caught fire, producing an explosion in which tens of local inhabitants were killed.
The accumulated corrosion failures at Dupont and Britoil were potentially costly and the two accidents were certainly so. That cost is ultimately borne by the community, but in the short term it falls on the industry concerned.
There is a growing awareness in industry of the costs of corrosion. This engenders a desire for more effort and expenditure on corrosion prevention and control. Industry finds willing allies in meeting this goal from the companies that sell anticorrosion materials and systems. Their products and services are not free.
THE COST OF CORROSION PREVENTION AND CONTROL
Expenditure on corrosion prevention and control is no less a tax on profits than the cost of corrosion itself. It is, therefore, entirely appropriate to ask if this expenditure is necessary.
An accountant can usually produce figures to illustrate the impact of a corrosion failure on profit. Some tangible, and less tangible, inputs to the calculation are given in Table 2.
Table 2. Cost of Corrosion Failure
Safety hazards Pollution clean-up costs
Loss of capital plant or equipment Increased insurance premiums
Fire/explosion Loss of consumer confidence
Loss of production capacity Alienation of workforce
Loss of product quality Increased scrutiny by statutory bodies Maintenance/repair/replacement Public image
Loss of stored/entrained product Accountants
By contrast, the accountant is rarely moved to make an assessment of the cost of not having a failure. If a plant and equipment operate without breaking down, everybody is usually well satisfied. It is rare to question whether the cost of achieving that performance has been excessive or even worthwhile. Table 3 lists some sources of possible over-spending in endeavouring to avoid corrosion failures.
Table 3. Costs of Over-Protection
Unnecessarily expensive materials Excessive data handling Overdesign of metal sections Overdesigned CP systems
Excessive weight Over-operated CP systems
Excessive inhibitor consumption Excessive replacement stocks Excessive monitoring Premature retirement of equipment Excessive inspection Over specified protective coatings
The items that relate to evident over-engineering in this list may be readily understood. However, two areas, monitoring and inspection, are worth singling out because they are so often regarded as a good thing, i.e., they have intrinsic merit. This is far from the case.
Industrial corrosion monitoring is commonly excessive both in terms of the extent of monitoring and the sophistication of the equipment used. We need to remind ourselves that corrosion monitoring has never controlled any corrosion. This author believes that corrosion monitoring should be used almost exclusively in a process control function, the process, in this case, being corrosion. Thus, a monitoring device should only be used in circumstances where the output from it can validly be used to adjust some corrosion controlling function, e.g., inhibitor injection or cathodic protection (CP) output. It follows that probes should not be installed where such action cannot be taken, nor should they be installed where probes
Plenary Lectures
installed elsewhere fulfil essentially the same function. In practice, these rules are observed more in their breach than in their application. Likewise, for reasons that are well understood, corrosion probes provide precise information on what is happening on the probe and, often, comparatively little about what is happening on the pipe or vessel wall. The value of the probe is that it detects change and prompts review and, possibly, action. Very often simpler means of detecting change than the use of corrosion-measuring devices will serve the same function at less cost, e.g., reference electrodes, pH probes, dissolved oxygen meters and moisture meters in the gas phase. They also have the merit, where relevant, of permitting continuous readout which allows the identification of the precise 'upset' that produces corrosion.
Inspection is similarly open to over-engineering. How often is inspection carried out because we have the opportunity ? It is remarkable that upon shutdown, the internal inspection of tanks and vessels will often be considered mandatory. Yet pipelines or pipework that carry the same fluids are not inspected. It has been pointed out [8] that the cost of inspection is high, often equivalent to 2-6% of the invested capital. That is a significant tax on profits. What is often overlooked is that inspection can often be potentially dangerous and may even produce conditions conducive to corrosion, e.g., when sulphuric acid tanks are opened up for inspection.
If corrosion costs money and corrosion control costs money, how do we target the optimum approach that strikes the correct balance between ignoring corrosion and seeking to control it ? The answer lies in
•
assessing the risk of corrosion failure on an item-by-item basis•
developing a lifetime corrosion management plan for each item to contain corrosion at an acceptable level of riskThis latter implies the need for risk modification, and sometimes, a defined degree of corrosion control activity. The goal is to maintain corrosion at an acceptable level. This begs the question: What is acceptable ?
RISK
It is a very dangerous game to talk about risk, largely because it is an ill-defined subject, and yet, everybody has a perception of what it is. What is clear is that risk is bad. We always associate risk with the likelihood of an undesirable or catastrophic event occurring. Thus, we talk of the risk of climbing, flying or crossing the road, but never of the risk of a traffic-free journey to work. Moreover, not everybody sees a particular risk in the same way. For example, we know that individuals are prepared to take a greater risk if they feel that they have some control over the process, or if the risk is associated with some activity considered to be beneficial [9]. That is why climbers do not appear to recognise the risk of climbing that is so self-evident to the rest of us. They feel a measure of personal control and perceive a personal benefit.
In short, risk is subjective. This is a worrying matter if, before we can proceed to a corrosion management plan, we need to make a corrosion risk assessment.
It is necessary to remove a little fuzziness. The most appropriate definition of risk is
Risk = Probability x Consequence (1) Despite its formality, this is not a very precise equation as we shall see. It does, however, indicate that risk is not simply a reflection of the probability of something bad happening.
Probability
Probability has the appearance of precision because it is a mathematical quantity. It derives from the stochastic nature of the frequency of the occurrence of events. Given sufficient failure data, a classic probability may be calculated to reflect the likelihood of a particular event occurring. In using the probability, it is important to be sure of its validity. Consider above-ground pipelines. The probability of failure, taking the overground pipeline population as a whole, is much less than the probability of failure of a small diameter (150-250 mm) line. Considerable error can arise from using the former probability in the latter case. Nevertheless, given valid and relevant failure data, a useful quantitative probability can be assessed.
Very commonly the failure data from which probability is calculated do not exist. The so-called Bayesian technique [10] can then be used to compute a probability. The technique uses prior knowledge (e.g., failure rates in similar, but not identical, circumstances elsewhere and the view of experts) and refines it steadily as specific information becomes available with plant operation. In the limit, of course, when the specific information database builds up sufficiently, the classic approach becomes more reliable.
Probability has one other unfortunate characteristic: uncertainty. The probability of an occurrence may be low, but it can happen tomorrow.
Consequences
The consequences element in Eq. 1 relates to the perceived magnitude of the loss if the failure occurs. This is a very subjective matter since different people rate the various consequences of an individual event differently, and many even disagree about the consequences that derive from that event. Nevertheless, it is possible for these individuals to list the potential consequences and to rate each consequence on a scale of 1-10 to compute a consequence for Eq. 1. The number that emerges is entirely subjective.
We see that the probability we use in Eq. 1 may hide a degree of uncertainty because of a lack of failure data, and it may include educated guesswork. Similarly, the consequence is a subjective valuation of the consequences of a failure. The outcome is a value for risk which, although numerical, is not exact.
CORROSION RISK ASSESSMENT
The foregoing does not seem to suggest that any form of risk assessment is likely to be productive. Yet the experience is that, in the case of corrosion, it can be helpful and rewarding.
In carrying out a corrosion risk assessment it is axiomatic that corrosion does not matter, but its consequences do. The assessment then aims to combine objective estimates of the possibility of a corrosion failure with the operating company's view of the level of
Plenary Lectures
undesirability of the consequences of it occurring. It will be seen below that the probability component of the risk is rendered quantitative and that the consequence component remains subjective, but reflects accurately the perceptions and ambitions of the people that own and run the plant. Subjective it may be, arbitrary it is not.
Methodology
The methodology of conducting a corrosion risk assessment has been discussed in detail elsewhere [11]. Only a brief outline is presented here.
If we take the example of a corrosion risk assessment in a refinery or chemical plant, it may be as coarse or as refined as the operator wishes. First, the plant is divided into systems, e.g., the gas sweetening unit. Second, each system is broken down into items. These usually comprise individual components, e.g., a vessel, a heat exchanger, a pump or a specifically identified length of pipework associated with the system. An item may be more widely defined in a coarse corrosion risk assessment or more closely defined in a fine analysis. In the latter case, for example, it may involve considering a vessel as a number of discrete items according to the known variation in fluid composition with height. Equally, it may be necessary to single out non-stressed relieved welds as separate items and, from an internal corrosion point of view, each dead leg.
What follows is an outline approach to corrosion risk assessment which has proved to be successful. Other methods are available that operate somewhat differently [12,13], but aim to achieve the same objective.
Life Factor
The aim of the corrosion risk assessment is to assign a risk number to each item using a risk equation similar to equation (1).
There are a variety of ways to deal with the probability element. The experience of the author's company is that it is best dealt with by assigning a life factor (L) that relates to the residual corrosion life of the item.
The residual corrosion life is the anticipated time required for corrosion at the predicted rate, or rates, to lead to failure to perform the required mechanical duty. Given information on the materials of construction, the exposure environment, and the relevant circumstances (e.g., temperature, pressure, flow, heat transfer, and stress), the morphology of corrosion can be predicted with confidence. Where uniform attack is expected, maximum penetration rates can be calculated using conventional corrosion engineering practices, including public domain algorithms [14,15,16], in-house database information and, if necessary, modelling. If localized corrosion is expected (e.g., pitting attack or one of the cracking modes of failure), probabilistic analyses of failure [17 ] are more useful.
For risk assessment purposes, the estimate of residual life in years is transposed to a dimensionless L. For example, an anticipated time to failure shorter than the time to the next shutdown would be assigned an L = 3. Anticipated lives beyond that point would attract L = 2, except where the residual life is put at >10 years in which case L = 1. Of course, this breakdown is arbitrary and the individual cut-offs, and the relative scoring, can be selected to match the requirements of the plant owners.
Consequence Factors
The point has been made that the consequence of a corrosion failure are more important than the failure itself. Thus, the consequences that bear on plant operators' minds include:
•
Safety,•
Production,•
Emergency repair,•
Operability,•
Environment,•
Third party interests,•
Customer perception, and•
Public perception.Adverse effects on any, or all, of these may often flow from an isolated failure.
The consequence factor (C) is a numerical assessment of the perceived consequences of a corrosion failure. The number is arrived at using structured group discussions with plant management, operations personnel, maintenance engineers, loss prevention officers etc. It elicits a subjective assessment. However, because individuals work towards a consensus in a group, and the methodology of subsequent analysis is rigorous, the rankings produced accurately reflect, in a quantitative way, the operating aspirations of the company concerned. Thus, the C numbers provide the relative importance attached to any consequence. Since the risk numbers that finally emerge are not absolutes but reflect perceived risk in a relative manner, the subjectivity of the consequence analysis is not only permissible, but desirable. The risk assessment becomes plant specific. That is, identical plants operated by different companies or in different locations will produce different risk assessment results.
There are two elements in establishing the value of C:
•
The individual consequence, and•
The events that can lead to that consequence.Some consequences will always be regarded by company personnel as more undesirable than others; to that extent, the staff can develop a point loading to be applied to each. This gives a consequence rating (F); a typical set to emerge in one case is given in Table 4.
Table 4. Typical Consequence Rating (f)
Consequence Rating (F)
risk of safety to personnel or public 10
loss of production 9
pollution 3
loss of produce quality 1
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It should be noted that the company in question is not concerned about contamination of the product by corrosion or any consequent alienation of the customer. This is a typical response from a primary producer; quite different numbers would have arisen in an assessment made in a food or pharmaceutical factory.
The events that lead to a given consequence produce an event rating (P). It is clear that a number of events which might occur in a process plant, may lead to the same consequence. The company staff are able to identify and rank these events according to their perceived undesirability, as shown in Table 5. In this case, the table relates to two plants owned by the same company in which one uses the product of the other.
Table 5. Typical Event Rating (P)
Consequence Event Rating (P)
Safety Crack in a toxic line or equipment 10
Pinhole in a melt line 10
Crack in a flammable line or equipment 9 Crack in other HP line or equipment 7 Pinhole in a flammable line or equipment 7 Pinhole in other HP line or equipment 6
Other cracks 5
Pinhole in toxic line or equipment 4
Other leaks 2
Falling objects 1
Outage Plant no. 1 - no standby 10
Plant no. 2 (HP) - no standby 9
Plant no. 2 (LP) - no standby 7
Plant no. 1 - standby 5
Plant no. 1 - non-critical - no standby 5
Plant no. 2 - standby 4
Plant no. 2 - non-critical 3
Plant no. 1 - non-critical - standby 3
Pollution Marine 10
Atmospheric 5
Quality Final product (colour only) 10
Intermediate product 5
It is not uncommon when considering safety, for staff to take into account the inventory of a system. Thus, they will commonly regard a crack in a system or item with a high inventory and, therefore, a high potential for damage, as more significant than one where the inventory is small. Different values of P may then arise according to the volume of an unisolatable part of the system. Any item included in the unisolatable part attracts the P value for that part.
The values of F and P are combined to yield C: 8
C = ∑Cx=
∑
xx==1n fn (Fx, Px) (2)Where the subscript x refers to each of the consequences, e.g., safety, pollution etc. in turn.
The Risk Equation
The risk equation must reflect the operating company's perception of risk associated with various forms and rates of failure. The equation, which is derivative of Eq. 1, produces a numerical assessment of risk (R) and takes the form:
R = fn (L, C) (3)
The shape of the function linking the life and consequence factors is determined by a formalized heuristic procedure. The function is modified through a series of computer iterations with the effect on the value of risk being assessed after each iteration.
Allocation of Risk Classes
The numerical value of risk can be calculated for each item in the plant. The higher the value of R for any item, the greater the risk and the more attention that must be focused on the local corrosion situation.
In practice, the spread of numerical risk values amongst all the items within a plant usually proves to be a discontinuous spectrum. That is, the risk numbers tend to fall into clusters with distinctive breaks between. This is an inevitable consequence of data like those recorded in Tables 4 and 5. It permits a convenient reduction of the numeric data into risk classes (e.g., low, medium and high). Such a sub-division aids communication of the outcome of the corrosion risk assessment either on a narrative basis or as colour coded P and ID's. It also assists with establishing a corrosion management programme.
Risk Modification
The fact that, in assessing a new or existing plant, areas of high risk have been identified, does not mean that the risk must be tolerated. The aim should be to moderate the risk and to move to a more acceptable condition.
Some methods of corrosion risk assessment [12] do not proceed as far as a risk equation or a risk number, but rather consider separately the perceived severity of the probability (L in this case) and the consequence (C). This produces a risk matrix as shown in Table 6.
Table 6. Risk Matrix
Consequence Probability
H M L
H H HM M
M HM M ML
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Risk categories can then be devised as shown in Table 7:
Intelligent risk modification aims to move towards Zone 3. This is not to aim at zero failure but to achieve, by good management, a tolerable level of risk.
Table 7. Risk Categories
High consequence High probability 1 High consequence Low probability 4 Consequence Low consequence High probability Low consequence Low probability 2 3 Probability
Using the risk equation approach, the aim is to concentrate resources on areas of high risk in order to reduce the risk number (i.e., modify the risk). Some care has to be taken here. It will often be the case that a high risk number will place the specific item in Zone 1 of Table 7. Clearly, for these items, it is important to move towards Zone 3 by means of corrosion control activities that reduce the risk number.
Somewhat lower risk numbers may fall into either Zone 2 or Zone 4. Indeed, the same risk number may apply to either a low consequence/high probability situation or a high consequence/low probability. The former simply represent failures that will be an irritation; pinholing in a seawater cooling line. The latter are certainly more serious; pinholing in a dry flammable gas line, for example.
In the case of Zone 2, the high probability means that sufficient data were available to assess the probability fairly accurately. By contrast, in the case of Zone 4, the reverse is true, and there may be considerable uncertainty.
If the probability (in our case, L) has been calculated using tried and tested tools, then identical risk numbers that derive from high probability/low consequence and low probability/high consequence events are equally reliable. Where the L calculation has used limited data, an uncertain algorithm or stochastic techniques, that level of reliability is absent. Thus, in moving from Zone 1 towards Zone 3, it is often better to achieve Zone 2 rather than Zone 4. Equally, it may be more important to move from Zone 4 than to move from Zone 2.
In general, the consequences of failure are usually not amenable to modification; thus, risk can only be modified by changing the probability. For that reason, and to introduce security, risk modification needs to pay attention to moving to situations where the probability is known or can be determined with some degree of confidence.
CORROSION MANAGEMENT
Corrosion risk assessment is not an end in itself. It identifies areas where corrosion may be safely ignored and where it must be attended to. It even provides the pointer to where resources will be spent with greatest reward. Thus, it provides the evidence that permits the construction of a cost-effective corrosion management programme. The objectives of a programme relate to the whole life of a facility and are to
•
Maintain corrosion within predetermined acceptable limits at minimum cost,•
Develop and facilitate rapid access to, records showing the corrosion status ofeach item within the facility in order to form a basis for future corrosion management decisions, and to provide assurance for managers, owners and statutory bodies, and
•
Ensure that corrosion upsets are quickly identified and appropriate remedial action is implemented, if necessary, to minimize the consequences of any failure.There is no universal corrosion management programme. Targeting these objectives is a unique exercise for every facility. However, the philosophy of corrosion management is common to them all. An overall strategy for corrosion management must first be agreed upon, and then the tactics become self-evident.
Strategic Considerations
The corrosion risk assessment will have produced a risk ranking for all items of a plant. This will enable a strategy for corrosion management to be set down. Table 8 illustrates a strategy that might be drawn up for an industrial facility.
Table 8. A Corrosion Management Strategy
Assessed Risk Alternative Corrosion Management Options High Corrosion prevention, or corrosion
control for life, or corrosion control to meet planned maintenance or planned replacement
Medium Corrosion control for life, or planned maintenance
Low No action, replace if required
It will be noted that corrosion prevention, or careful corrosion control, is dictated by a high risk classification. By contrast, a low risk classification justifies no corrosion controlling action. A medium risk requires some action. Thus, corrosion management involves a spectrum of activity from no action to considerable action according to the risk. However, taking no action, or taking action, is not corrosion management unless the decision to follow the particular course has been based on an assessment of risk. Action where it is not needed, like inaction where it is, represents a waste of resources and a tax on profits.
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It will be recalled that corrosion risk assessment is carried out by dividing the plant into systems and items. Ultimately, the output relates to individual items. It is possible for an item within a system to have a high risk classification whilst other items in the same system belong to a lower risk class. The decision must then be made whether to apply corrosion control to the system in order to preserve the item, or to ensure corrosion prevention for the item (say, by the use of more corrosion-resistant material) and avoid dealing with the system. The application of the broad strategy does allow, and requires, some flexibility in the tactics adopted.
Tactical Considerations
The complete elimination of the chance of corrosion failure, i.e., corrosion prevention, in a high risk area is rarely possible in an existing plant. Invariably, it would require a significant engineering change, for example, replacement of existing materials by corrosion-resistant alloys or modification of the process (e.g., addition of gas dehydration). Even with new plants, such proposals might raise major design and engineering problems, not to mention cost.
It is much more likely that active corrosion control will be adopted with the objective of extending the time to failure of an item beyond the planned life of the plant, or up to some planned maintenance shutdown. The adoption of this tactic requires that:
•
the performance targets for the corrosion control are defined, and•
procedures are put in place to ensure the targets are met.The performance target may be set in terms of an allowable rate of metal penetration. This approach will most commonly be adopted when uniform corrosion is anticipated. Alternatively, limits may be set on some parameter that is an indication of fluid corrosivity, e.g., electrode potential in anodic and cathodic protection systems, dissolved oxygen in oilfield water injection or boiler feedwater, pH, temperature, or dewpoint. Irrespective of which approach is adopted, it will be necessary to obtain on-line information to make adjustments as required. Thus, corrosion monitoring is necessary, and it then forms an essential segment of the corrosion management plan. Table 9 lists some monitoring techniques and indicates how they may be used in corrosion management.
Table 9. Corrosion Monitoring in Corrosion Management
Corrosion Control Strategy Monitoring Technique Examples of Adjustments and Activities Based on Data Monitoring
Inhibition of crude oil pipelines
On-line probes (e.g., coupons, electrical resistance probes)
Adjust inhibitor dosage, change inhibitor type, discontinue inhibition De-oxygenation of boiler
feed-water
O2probes Adjust oxygen scavenger,
check pump seals, etc. Impressed current CP Potential Adjust system output Anodic protection of
sulphuric acid plant
Potential Adjust system output
Dehydration of process gas On-line probes, moisture detection
Temporary inhibition, overhaul dehydrator
The key to effective corrosion management is information since it is on the basis of that information that on-going adjustments to corrosion control are made. Information is valid data. Thus, to make effective corrosion management decisions on a day-to-day basis, the monitoring data must be valid. This is not simply a requirement for the probes to be operating correctly. It requires that they be placed in the most appropriate places, i.e., at those points where the corrosion controlling activity might be expected to work, but where it might equally be expected to be least effective, e.g., remote from the inhibitor injection point. In many cases specially designed traps are introduced into a plant so that corrosion probes may be inserted. These often produce their own microenvironment, atypical of the plant itself, and with little hope of effective entry for an inhibitor. Data from a probe in such a location are unlikely to be relevant to corrosion management elsewhere in the system. Invalid data leads to ineffective corrosion management.
Keeping Track
In any facility the means of corrosion management will vary from place to place. In one location a corrosion resistant alloy may be used; in another, CP allied to coating may be employed, whilst elsewhere no action may be taken because the consequences of any failure are regarded as unimportant. In short, no corrosion management action is taken that does not contribute positively to meeting the objective of containing risk whilst maintaining the level of action at the minimum necessary.
It is important to ensure that the targets are being met. Overshooting the target will involve excessive corrosion control costs, whilst undershooting the target may lead to a situation that cannot economically be recovered. Corrosion monitoring is not appropriate for the purpose since it rarely provides evidence of the metal loss from a pipe or vessel wall. That is, aggregating the output from probes over time does not give any indication of the loss of a section. The value of corrosion probes is that we rapidly develop experience so that we can be reasonably sure that when the probes read a given value, we are on target, and that a change in reading requires consideration of an adjustment to the corrosion controlling activity. Reference electrodes, pH probes, moisture meters etc. often fulfil the same function.
Thus, corrosion probes and the like, do not provide quantitative performance assessment. That can only come from inspection and nondestructive testing (NDT). These activities are part of corrosion management since they provide reassurance, identify wasteful corrosion control activity, and permit reassessment of the corrosion management programme. The same critical approach that was adopted in setting up the corrosion control strategy must be applied to the inspection strategy. That is, the resources must be applied according to the risk. If we have attempted to modify the risk by instituting some corrosion control activity, we should be tracking the success, or excess, of the activity in our inspection programme. Thus, inspection is not based on convenience, inspecting because an item is accessible (at shutdowns, for example). It should be based on the premise that if the consequences of failure are to be avoided and the cost of control is to be minimized, inspection is necessary. There must, therefore, be a clear connection between the risk assessment output, the corrosion management strategy, the tactics of corrosion management and the inspection programme.
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The key point approach to NDT is particularly effective. Here a limited number of points, in areas where validation is required, that are regarded as typical and extreme, are identified for NDT inspection at regular intervals. This provides, on a temporal basis, a readout of the progress of corrosion which will validate, or otherwise, the targeting achieved by, say, inhibitor injection.
Similarly, during internal inspection, the risk assessment will have identified particular areas of concern, e.g., tube/tube sheet assemblies, tube baffles, and non-stress relieved welds, which must become the focus of activity. Again, this will confirm whether the corrosion control is adequate or perhaps is insufficient or excessive.
The data that are produced from the inspection activities must be valid and limited in volume so as not to deter analysis or hide anomalies. Thus, it is important to restrict key points and inspections to critical positions and to limit the frequency of inspection and survey work. The time to the next inspection or survey should be indicated by the outcome of the current work. That is, a lifetime fixed interval programme will usually prove wasteful; inspection and survey should be carried out on an as-needed basis. A valuable template giving an approach to the re-classification of in-service inspection is to be published in 1996 [18] and has been reviewed in reference [12].
Review
From time to time a corrosion management programme should be reviewed at both the strategic and tactical level. In human affairs, things change. The management of a facility will always be alive to current market trends, competitors activities, interest rate movements and so on. Inevitably, it may be necessary to revise the management objectives from time to time. Since the corrosion management programme was constructed to meet the objectives of an earlier plant management plan, it will be necessary to review the programme and possibly to alter it. Likewise, the pace of technological change is rapid compared to the anticipated lifetime of most facilities. Thus, newer, more effective, cheaper means of achieving the same ends may emerge, and indeed, it may be possible to adopt them in place of existing tactics within the corrosion management programme. Thus, the programme is not a fixed blueprint, but a means to an end that must be reviewed and revised to meet the current management objective.
One objection that is raised to corrosion management planning comes from the corrosion engineers themselves. They draw attention to the fact that by fixing permissible rates of metal loss, the lifetime of the facility is effectively determined. Further, that management will often, at a later stage, decide to extend the required operating life. There is then a mismatch. The argument seems to be that corrosion management planning should ignore the present requirements and anticipate the future requirements of the management. This is an extremely wasteful approach. Certainly there is a possibility that a mismatch will occur and will need to be overcome. That will be achieved at some cost. That cost must be attributed to the decision to go for life extension and is, therefore, a natural consequence of that extension. It needs to be included in the cost benefit analysis of extension, not hidden in lifetime overspending in anticipation that life extension might be required. It may not be.
ILLUSTRATIONS
Two recent examples illustrate how corrosion risk assessment provided important results for the clients. In the first instance, the assessment of a petrochemical complex in the Middle East found the plant to be extremely well engineered from the corrosion standpoint. It was constructed predominantly in carbon steel, with excursions into more exotic metallurgy only where the process conditions demanded it. However, the assessment highlighted, somewhat to the client's surprise, the cooling water system as a high risk area.
By using a closed system with secondary cooling by seawater and specifying high quality primary water with corrosion inhibitor injection, the designers had judged that it was possible to construct the majority of the cooling system in carbon steel. Certain that the primary heat exchangers were, however, constructed in a stainless steel due to the aggressivity of the process fluid, the corrosion risk assessment identified modes whereby the quality of the cooling water could be adversely affected (e.g., by leakage of seawater at secondary plate exchangers). Failure to maintain cooling water within specification would very rapidly lead to stress corrosion cracking of one of ten critical stainless steel process exchangers, failure of any one of which would halt production.
In view of this, Global Corrosion put forward recommendations for modest on-line monitoring of cooling water quality. Tied to this was the setting up of a formal action plan to be followed in the event a sudden deterioration in water quality should be detected. The client accepted and implemented these recommendations but, unfortunately, not before one failure of the type predicted occurred.
The second example derives from an installation, also in the Middle East. The corrosion risk assessment concluded that the absence of CP on water storage tanks, together with the prevailing soil conditions, would result in high tank bottom corrosion rates. Since an adequate supply of water was essential to maintain production, the assessment concluded that the prospective failure of the tanks constituted a high risk and it was strongly recommended that CP be installed.
In the event the client was reluctant to accept and act upon the outcome of the report. Just under a year later the raw water tank perforated due to soil-side corrosion. The resulting loss of water caused a two week interruption in production, prompted a belated decision to install CP and engendered in the client a heightened appreciation of the benefits of corrosion risk assessment and the need for effective corrosion management.
CONCLUSIONS
Corrosion cannot be ignored for it will not go away. However, there is little merit in controlling corrosion simply because it occurs, and none in ignoring it completely. The consequences of corrosion must always be considered. If the consequence of corrosion can be lived with, it is entirely proper to take no action to control it. If the consequences are unacceptable, steps must be taken to manage it throughout the facility’s life at a level that is acceptable. To manage is not simply to control.
Good corrosion management aims to maintain, at a minimum life cycle cost, the levels of corrosion within predetermined acceptable limits. This requires that, where appropriate, corrosion control measures be introduced and their effectiveness ensured by judicious, and not excessive, corrosion monitoring and inspection. Good corrosion management serves to support the general management plan for a facility. Since the latter changes as market
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conditions, for example, change, the corrosion management plan must be responsive to that change. The perceptions of the consequences and risk of a given corrosion failure may change as the management plan changes. Equally, some aspects of the corrosion management strategy may become irrelevant. Changes in the corrosion management plan must, inevitably, follow.
REFERENCES
1. T.P. Hoar (Chairman), Report of the Committee on Corrosion and Protection, HMSO, London, 1971
2. B.W. Cherry and B.S. Skerry, Corrosion in Australia - the Report of the Australian National Centre for Corrosion Prevention and Control Feasibility Study, Monash University, 1983.
3. L.H. Bennett, National Bureau of Standards Special Publication 511.1, NBS, Washington, 1978.
4. A.J. Sedriks, Corrosion of Stainless Steels, Wiley, 1979, p. 7.
5. Kermani, An Overview of Wet H2S Attack: Types, Causes and Problems, in Papers of
the Conference on Wet H2S Attack on Steels, Institute of Mechanical Engineers,
London, 1996.
6. F. Lees, Loss Prevention in the Process Industries, Vol 2, p863, Butterworth (1989) 7. J.M. Malo, V. Salinas and J. Uruchurtu, Materials Performance 33, 8, 1994, p. 63. 8. C. Edeleanu and J.G. Hines, Materials Performance 29, 12, 1990, p. 68.
9. P. Slovic, Science 236, 17 April 1987, p. 280.
10. M.E. Giuntini, Proceedings of Fourth Space Logistics Symposium, Florida, November 1992.
11. V. Ashworth and W.R. Jacob, Proc. Corrosion 32, Australasian Corrosion Association, 1992.
12. B. Spalford, Carbon steel equipment in wet H2S service, Papers of Conference on Wet
H2S Attack on Steels, Institution of Mechanical Engineers, London, 1996. 13. Private communication, Shell-Expro, UK
14. C. de Waard, V. Lutz and D.E. Milliams, Corrosion 47, 1991, 976. 15. F.A. Posey and A.A. Palko, Corrosion 35, 38 (1979)
16. J.W. Oldfield, G.L. Swales and B. Todd, Proc. 2nd BSE/NACE Corrosion Conference, Bahrain, 1981.
17. M. Akashi, Proc. Conference of Life Prediction of Corrodable Structures, NACE, 1991.
18. EEMUA publication 179, A Working Guide for Carbon Steel Equipment in Wet H2S
Service (to be published in 1996)
THE DETERMINISTIC PREDICTION OF DAMAGE
D.D. Macdonald
Center for Advanced Materials The Pennsylvania State University
517 Deike Building, University Park, PA 16802, USA
ABSTRACT
As our industrial and infrastructural systems (refineries, power plants, pipelines, etc.) age, considerable economic incentive develops to avoid unscheduled outages and to extend operation beyond the design lifetime. The avoidance of unscheduled outages is of particular interest, because the failure of even a minor component can result in the complete shutdown of a facility. For example, the unscheduled shutdown of a 1000 Mwe nuclear power plant may cost the operator between US $1 million and US $3 million per day, depending upon the cost of replacement power and other factors. However, if component failures could be accurately predicted, maintenance could be performed during scheduled outages, the cost of which has already been built into the price of the product. With regard to life extension, the successful extension of operation beyond the design life translates into enhanced profits and the avoidance of costly licensing and environmental impact assessments associated with the development and construction of a new facility. In this case, as well, the key to successful operation is the ability to avoid downtime, and hence, to maintain production. Eventually, the frequency and severity of unscheduled outages will render operation uneconomic, and at that point, replacement of the facility is necessary. In order to develop effective inspection and maintenance scheduling and life extension technologies, it is first necessary to predict the evolution of damage into the future as a function of various system variables. The only effective prediction technologies are those based on determinism, in which the system behavior is described in terms of natural laws. In this paper, the deterministic prediction of damage, via damage function analysis (DFA), which provides a robust technology for estimating the damage function at future times, is described. The application of DFA to the prediction of pitting damage is illustrated by reference to pitting damage in condensing heat exchangers.
Key Words: Corrosion damage, determinism, prediction, pitting corrosion.
INTRODUCTION
Corrosion is a major cause of component failure, and hence, in the occurrence of unscheduled downtime, in complex industrial systems. In particular, the various forms of localized corrosion, including pitting corrosion, crevice corrosion, stress corrosion cracking (SCC), and corrosion fatigue (to name the common forms) are particularly deleterious because they frequently occur without any outward sign of damage, and because they often result in sudden and catastrophic failures. Thus, the development of effective corrosion damage prediction technologies is essential for the successful avoidance of unscheduled downtime and for the successful implementation of life extension strategies.
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Corrosion damage is currently extrapolated to future times using damage tolerance analysis (DTA). In this strategy, known damage is surveyed during each subsequent inspection, and the damage is extrapolated to the next inspection period allowing for a suitable safety margin. We have argued [1] that this strategy is inaccurate and inefficient, and that in many instances it is too conservative. Instead, we argue that damage function analysis (DFA) is a more effective method for predicting the progression of damage, particularly when combined with periodic inspection. DFA is based upon the deterministic prediction of the rates of nucleation and growth of damage, with particular emphasis on the compliance of the embedded models with natural laws. Although corrosion is generally complicated mechanistically, a high level of determinism has been achieved in various treatments of both general and localized corrosion.
The application of DFA is illustrated by reference to the development of damage due to pitting corrosion of stainless steels in condensing heat exchangers. Deterministic models have been developed for both the nucleation and the growth of damage, and these models have allowed us to calculate the damage function as a function of exposure time and system conditions.
FUNDAMENTAL CONCEPTS
In this paper, I outline a deterministic method for predicting the damage function for pitting corrosion in condensing heat exchangers [1,2]. This method is considered to be potentially superior to empirical (including stochastic and probabilistic) techniques, because it is mechanistically-based and hence provides analytical relationships between the damage function (i.e., number of pits versus pit depth presented in the form of a histogram [1]) and the damaging variables (e.g., chloride concentration and combustion parameters). Accordingly, deterministic methods are expected to be more efficient at using databases, because a lesser need exists to establish the damage function/damaging variable relationships empirically.
Any deterministic model must account for the fact that localized corrosion involves nucleation and growth phenomena which occur sequentially for a single site but that tend to occur in parallel for an ensemble of pits. Furthermore, the model must account for the experimental observation that the parameters that characterize the breakdown event are distributed, due to the fact that the population of sites on any real surface is not homogeneous. Outlined below is one model that satisfies these (and many other) conditions related to the nucleation and growth of damage resulting from localized corrosion. While the model may not be complete (or even correct), it is deterministic in that the distribution function and the relationships between the model parameters and the damage function are analytic and follow from the natural laws. In illustrating this technology, I have chosen to discuss the prediction of the damage function for pitting corrosion, because this form of attack is almost ubiquitous in condensing heat exchangers. Furthermore, pitting corrosion displays most of the features of all forms of localized attack, including an induction time and the autocatalytic development of the damage.
The algorithm developed in this study to estimate the damage functions for condensing heat exchangers contains five modules as outlined in Fig. 1. Also indicated are the parameters that propagate from one module to the next. The output of the algorithm can be specified in three forms:
•
For a specified probability of failure, the algorithm estimates the damage function as a function of exposure time and computes the number of pits with lengths exceeding the condenser wall thickness to predict the service life.•
For a specified probability of failure and design life, the algorithm calculatesthe wall thickness to ensure acceptable performance.
•
For a specified wall thickness and design life, the algorithm calculates the failure probability. MODEL INPUTS Duty Cycle Chloride Concentration Condensate Temperature Flue Gas Composition Condensate Chemistry Model Mixed Potential Model Pit Nucleation Model Pit Growth Model Damage Function ModelService Life Wall Thickness
Specifier Failure Probability pH.[Cl-]* Ecorr.pH. [Cl-] N(t).Ecorr.pH. [Cl-] N(tobs) vs. n(u)
Figure 1. Structure of the algorithm for the prediction of damage function (*parameters propagated from one model to the next)
Below I describe the various modules in this algorithm; however, due to the limited space available, I outline only the principles of these modules.
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The Condensed Chemistry Module (CCM)
The composition of the flue gas will differ from burner to burner. With this in mind, we developed a generalized condensate model for the condensate environment. This model assumes the flue gas to be a mixture of CO, CO2, H2S, NO, NO2, SO2, SO3, and H2O. The relative proportions of these components may vary widely from furnace to furnace, depending on the nature of the ambient air, the air/gas ratio, and the impurities of the gas. The goal of the condensed chemistry module (CCM) is to calculate the pH and the composition of the condensate on the condenser surface. The pH is a key parameter in controlling the rates of pit nucleation and pit growth. The concentrations of species in the liquid layer determine the ionic conductivity of the solution, which has great impact on the pit growth rate. The module employs an equilibrium model along with mass balance and charge balance constraints, and computes ion activity coefficients using the extended Debye-Huckel theory. It is assumed that the condensed liquid film is in equilibrium with the ambient environment, so that equilibrium calculations are applicable. The details of this module are described in the literature [3].
A typical gas-fired heat exchanger is schematically shown in Fig. 2a [4]. The temperature ranges from approximately 308oK in the cold end to 353oK in the hot end, depending on the design of the heat exchanger. Typical values of the pH and chloride concentration in these different zones are given in Fig. 2b [4]. It is shown that the condensed liquid phase is enriched in chloride to the extent of approximately 150 ppm in the hot end. Acidification of the condensed thin liquid layer also occurs, in that pH values as low as 2.7 and 3.3 are found at the hot end and the cold end, respectively. In Fig. 2c, the computed pH for a typical composition of the flue gas and the chloride content of the condensate are presented. The calculation shows a variation in pH from 2.93 to 3.32 from the hot end down to the cold end. Recognizing the wide range of operating conditions and designs of condensing heat exchangers, it is concluded that good agreement is observed between the experimental data and theoretical prediction.
Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 Flue Gas Flow Cooling Air Exhaust Heat Exchanger Simulator T = 353-326o K T = 308-326o K Cooling Air
Figure 2a. Schematic diagram of a typical heat exchanger in a gas-fired furnace [4]
Figure 2b. Characteristics of flue-gas condensate from different zones [4]
Figure 2c. Calculated pH in the condensate as a function of temperature and chloride concentration (PCO = 2.60 x 10-5 atm, PCO2 = 6.80 x 10-2 atm, PSO2 = 9.37 x 10-9 atm, PSO3 = 2.00 x 10-9 atm, PH2S = 1.20 x 10-4 atm, PNO = 1.74 x 10-8 atm, PNO2 = 2.67 x 10-8 atm, PO2 = 1.00 x 10-4 atm)
The Mixed Potential Module (MPM)
The mixed potential module (MPM), which is based on the Wagner-Traud hypothesis [5] for free corrosion processes, was developed to calculate the corrosion potentials of alloys in corrosive environments. The theory outlined here is essentially identical to that developed by Macdonald et al. for calculating corrosion potentials for stainless steel components in the heat transport circuits of boiling water reactors (BWRs) [6,7]. The theory is based on the physical condition that charge must be conserved in the system.
