Stainless Europe
What are the major factors of corrosion?
What are the 5 principal types of corrosion linked with the surrounding environment ?
What is corrosion?
Metals, with the exception of the precious metals such as gold and platinum, that are found in their natural state are always extracted from ores; metals have therefore a tendency to revert to their stable state, which corresponds to their original state, that is to say their oxide form. Metal corrosion is essentially an electro-chemical reaction at the interface between metal and surrounding environment . Stainless Steel and the passive layer
Steel is an alloy of iron and carbon. Contrary to carbon steel, the presence of a minimum of 10.5 % chromium in the stainless steel gives it the property of corrosion resistance.
Indeed, on contact with oxygen, a chromium oxide layer is formed on the surface of the material. This passive layer protects it and has the particular ability to self repair.
However if this protective layer is damaged, the start of corrosion can appear
Stainless Steel and
Corrosion
The Composition of Stainless Steel
It is the chromium which gives the stainless quality to our steels.
Iron Chromium >10,5% Fe+C=steel Fe+C+Cr=stainless steel Carbon <1,2% O2 Formation of chromium oxide Stainless Steel Formation of iron oxide (rust) Steel Rust Passive layer
Reaction of steel and stainless steel in contact with moisture in the air or water.
Fe + C + Cr > 10,5 % Fe + C MediuM Material design tiMe Corrosion faCtors
Ageing of the structure Evolution of stresses Temperature variability Modification of coatings Maintenance frequency Composition Manufacturing Metallurgical state (thermal or mechanical treatment) Additives Impurities Surface state Shape Assembly (welds, rivets) Mechanical stresses Proximity to other metals Contact with a medium (partial or total immersion) Methods of protection Chemical nature Concentration Oxidising power pH (acidity) Temperature Pressure Viscosity Solid deposits Agitation generalised generalised loCalised under stress CreviCe Pitting inter -granular neutral Chloride environMent aCidMediuM
Pitting Corrosion
This dissolution gives rise to metal ions and electrons and thus the passage of current (of dissolution) which gives rise to an electrical potential difference between the anodic zone (pitting) and the cathodic zone (the rest of the metal).
Figure 1 shows pitting potentials obtained for different stainless steels in water containing 0.02M NaCl (710mg/l Cl-) at 23°C.
It shows the influence on the resistance to pitting corrosion with the content of chromium and molybdenum for
the ferritics, and chromium, molybdenum and nitrogen content for the austenitics.
Generalised corrosion is noticed when stainless steel is in contact with an acid medium and localised corrosion is seen in the majority of cases when stainless steel is placed in a neutral chloride environment
In the document which follows we describe the 5 principal types of corrosion and we rank the majority of Stainless Europe grades from standard laboratory tests
However, the phenomena of corrosion in real life are always specific, the data described does not exclude extra trials to choose the optimal material.
To understand the phenomenon
Pitting corrosion is a local break in the passive layer of the stainless steel provoked by an electrolyte rich in chloride and or sulphides. At the site of the pitting, where the metal is unprotected, corrosion will develop if the pit does not re-passivate, in other words if the speed of metal dissolution enables to maintain a sufficiently aggressive environment to prevent its re-passivation.
If the potential of the stainless steel in the given medium is superior to the pitting potential =>the stainless steel corrodes
Note: the higher the pitting potential, better shall be the corrosion resistance of the grade. Outside the pits, the passive layer is always present to protect the stainless steel.
The pitting potential corresponds to the potential necessary to initiate stable pits.
If the potential of the stainless steel in the given medium is inferior to the pitting potential => pitting does not start
2nd case Chlorides Passive layer Metal passive layer Metal
The metal re-passivates Corrosion of the base metal METAL ATTACK
Passive layer Metal
To simulate this type of corrosion under laboratory circumstances, a sample is immersed in a corrosive electrolyte to which an increasing potential is applied until the passive layer is broken.
During this dynamic potential (intensity/potential) scan the sudden increase in intensity corresponds to the pitting potential Epitting INTENSITy I =50µA/cm2 POTENTIAL Passive fi eld Epitting 800 700 600 500 400 300 200 10 12 14 16 18 20 22 24 26 Martensitic K39 mV/SCE PREN (%Cr+3,3%Mo+16%N) P itt ing p ot en tia l in w at er co nt ain ing 0 .0 2M Na CI p H= 6, 6 at 2 3° C (m V/ SC E ) K41 Figure 1 K45 K44 K36 K30-K31 K03 K09 K10 17 -4 M n 16 -4 M n 16 -5 M nL 18-7L 17-7A/C/E 18-10T 18-9E/L 17-11MT 18-11ML Commercial designations Norms EN ASTM Type Martensitic 420 1.4021 K09 409 1.4512 K03 1.4003 K10 410S 1.4000 K30-K31 430 1.4016 - 1.4017 K39 439 1.4510 K41 441 1.4509 K36 436 1.4526 K45 445 1.4621 K44 444 1.4521 16-4Mn 201.2 1.4372 16-5Mn 201LN 1.4371 17-4Mn 201.1 1.4618 17-7A/C/E 301 1.4310 18-7L 301LN 1.4318 18-9L 304L 1.4307 18-11 ML 316 - 316 L 1.4401 - 1.4404 17-11MT 316Ti 1.4571 Standards
As figures 2 and 3 show, this pitting potential can only be used to rank the grades in a given medium.
It diminishes notedly when the temperature (figure 2) or the concentration of chlorides in the medium increases. (figure 3)
Our Recommendation To avoid pitting corrosion:
We would look to see if it is possible to lower the corrosiveness by lowering the temperature of the medium, limiting contact time, avoiding stagnant areas and reducing the concentration of halogens and the presence of oxidants.
We would choose a grade high in chromium or containing molybdenum.
Usually we use the PREN (Pitting Resistance Equivalent Number) of the grades to rank their general piting behaviour. The PREN, %Cr+3.3%Mo+16%N , demonstrates the major influence of these alloy elements.
To understand the phenomenon A/ Initiation of corrosion
In an electrolyte high in chloride, a confined (occluded) zone linked for example to bad design, favours the accumulation of chloride ions. The progressive acidification of the medium in this zone facilitates the de-stabilisation of the passive layer. When the pH in this zone reaches a critical value called « depassivation pH »,corrosion starts. The depassivation pH or pHd is used to characterize the resistance to crevice corrosion initiation.
Figure 3 Figure 2 500 450 400 350 300 250 200 150 100 50 0 10 12 14 16 18 20 22 24 26 K30 K39 K41 18-9E/L 18-11ML K44 K03-K09-K10 K36 Pi tt ing p ot en tia l in w at er co nt ain ing 0 .5 M N aC I p H= 6, 6 at 5 0° C (m V/ SC E) PREN (%Cr+3,3%Mo+16%N) 700 600 500 400 300 200 100 0 10 12 14 16 18 20 22 24 26 K36 K30 K39 K4118-9E/L 18-11ML K44 K03-K09-K10 Pi tt ing p ot en tia l in w at er co nt ain ing 0 .0 2M N aC I p H= 6, 6 at 2 3° C (m V/ SC E) PREN (%Cr+3,3%Mo+16%N)
Crevice Corrosion
HOW? Break in the passive layer and metal attack WHy?Confined zone (acidification)
In order to map the duplex range, tests in the most severe medium, 0.5M NaCl (17.75g/l Cl-) at 50°C, were carried out. The results obtained are shown below.
Figure 4 Pi tt ing p ot en tia l in w at er co nt ain ing 0 .5 M N aC l, a t 2 3° C an d 50 °C (m V/ SC E) Appellations commerciales Normes ASTM EN Designation Type UNS DX2202 2202 UNS 32202 1.4062 DX2304 2304 UNS 32304 1.4362 DX2205 2205 UNS 32205 1.4462
Some pHd values for our stainless steels are given in figure 5. The lower the value pHd the better the resistance to crevice corrosion.
If on a recording we detect a current peak (activity), crevice corrosion is starting , in the opposite case repassivation takes place.
Activity peak measurement for a pH lower to the depassivation pH can then be considered to quantitatively compare the speed of crevice corrosion propagation for different grades. This value is sensitive to the alloy elements which improve the passivity and limit active dissolution, principally molybdenum, nickel and chromium (see figure 6 ).
The speed of propagation is also a function of local aggressiveness and temperature of the medium.
B/Propagation of corrosion
Once corrosion is initiated, its propagation occurs by active dissolution of the material in the crevice.
In the laboratory, we simulate this type of corrosion by recording the potentiodynamic scans in chloride mediums of increasing acidity.
Our Recommendation
Our first recommendation to avoid crevice corrosion is to optimise the design of the piece to avoid all artificial crevices. An artificial crevice can be created by a badly made joint, a rough or bad weld, deposits, gaps between two plates etc.,
If the confined zone is unavoidable, it is preferable to enlarge this zone and not to make it smaller.
If the design of the pièce is not modifiable or if the fabrication process makes difficult to avoid confined zones, the risk of crevice corrosion is very high . We recommend, in this case, choosing an appropriate grade, in particular a stainless steel austenitic or duplex when the product will be in contact with corrosive media or part of the process equipment.
Intergranular corrosion
To understand the phenomenon
At temperatures greater than 1035°C , the carbon is in solid solution in the matrix of the austenitic stainless steels. However, when these materials are cooled slowly from these temperatures or even heated between 425 and 815 °C, chromium carbides precipitate at the grain boundaries. These carbides have a higher chromium content in comparison to the matrix.
Consequently, the zone directly adjacent to the grain boundaries is greatly impoverished.
The sensitisation state takes place in a lot of environments by privileged initiation and the rapid propagation of corrosion on the de-chromed sites.
For unstabilized ferritic stainless steels, the sensitisation temperature is greater than 900°C.
Figure 5: Depassivation pH of various stainless steels in NaCl 2M (71g/l Cl-) de-aerated and acidified with HCl at 23°C
4 3,5 3 2,5 2 1,5 1 0,5 10 15 20 25 30 35 PREN (%Cr+3.3%Mo+16%N) 2500 2000 1500 1000 500 0 1 3 5 7 9 11 13 0.2%Cr+%Mo+0.4%Ni
Figure 6: Critical current «icrit» at the peak of activity for various stainless steels in NaCl 2M (71g/l de Cl-) deaerated and acidified with HCl at 23°C.
Martensitic K03/K09/K10 K30/K31 K41 K39 K45 DX2202 187L 177A 1898 1810T1810L 1811L1711MT DX2205 1812MS 1813MS DX2304 164Mn 165MnL 174Mn K44 D ep as siv at io n pH o f v ar io us st ain les s s te els in N aC l 2 M ( 71 g/ l C l -) de -a er at ed an d ac id ifi ed w ith H Cl a t 23 °C C rit ica l c ur re nt « icrit » at th e pe ak o f a ct ivi ty fo r v ar io us st ain les s s te els in N aC l 2 M (7 1g /l de C l -) d e-ae ra te d an d ac id ifi ed w ith H Cl at 2 3° C.
Our recommendation
In practice , this case of corrosion can be encountered in the welded zones . The solution for the austenitic consists of using a low carbon grade called « L » (Low C%<0.03%) or a stabilized grade, and the titanium or niobium stabilized ferritic grades.
The volume of the piece permiting a thermal treatment of the quenching type (rapid cooling) at 1050/1100°C or a tempering of the welded piece can be done.
Stress Corrosion
To understand the phenomenon
We mean by « stress corrosion » the formation of cracks which start after a period of long incubation and which afterwards can propagate very rapidly and provoke downtime of the equipment by cracking .
This particularly dangerous phenomenon is the result of the combined effects of 3 parameters:
- temperature, since stress corrosion rarely develops under 50°C
- the applied or residual stresses
- the corrosiveness of the medium : presence of Cl-, H2S or caustic media NaOH
Although stress corrosion of ferritics can be provoked by particularly aggressive tests in the laboratory ,
their body cubic centred structure rarely renders them subject to this type of phenomena in practice .
The face cubic centred structure of austenitic stainless steels can present a risk.
In effect , it favours a mode of planar deformation which can generate very strong stress concentrations locally. As shows the graph below, this is particularly true for classic austenitic stainless steels with 8% nickel; an increase in nickel above 10% is beneficial .
In austenitic stainless steels, the austenitic stainless steels with manganese perform worse.
The austeno-ferritic structure of the duplex gives them an intermediate behaviour, very close to the ferritics in the chloride medium and even better in the H2S medium .
The metallurgical structure of stainless steels influences their behaviour in this type of configuration CRACKS Chlorides Temperature effects STAINLESS STAINLESS PASSIVE LAyER Aggressive medium PASSIVE LAyER STAINLESS STEEL STRESS 1000 100 10 1 0 20 40 60 80 Nickel content, wt.% No cracking Cracking Ti m e t o cr ac k ( ho ur s)
Effect of nickel content on the resistance to stress corrosion of
Stainless steel containing from 18-20% chromium in magnesium chloride at 154°C [ From a study by Copson [ref] . Physical Metallurgy of Stress Corrosion Cracking, Interscience, New York, 247 (1959).]
Our recommendation
To avoid this type of corrosion :
Suppress the stresses or have a better redistribution, by optimising the design or by a stress relieving treatment after forming and welding of the pieces concerned.
lower the temperature if possible
If not practicable, choose the grade most adapted, favouring as a solution a ferritic or duplex but bearing in mind the other corrosion problems encountered .
Uniform corrosion
To understand the phenomenon
This is the dissolution of all the affected points on the surface of the material which are attacked by the corrosive medium. On the micrographic scale,this corresponds to a regular uniform loss of thickness or loss of weight (uniform or generalised corrosion as opposed to localised corrosion). We see this corrosion in acid mediums. Indeed, below a critical pH value, the passive layer protecting the stainless steel is no longer stable and the material suffers a generalised active dissolution. The more acid the medium, the faster the corrosion and the loss of thickness of the stainless steel.
In the laboratory, we measure this speed of corrosion in an acid medium by graphing the polarisation curve (see below). An increasing potential scan is imposed on the metal and the corresponding intensity is recorded.
The maximum current reading of the activity peak allows us to classify the resistance of different grades to this type of corrosion (see figure 7).
Generally, the higher the current, the faster and greater the dissolution, thus the less the grade will be resistant.
Head office ArcelorMittal Paris
Stainless Europe 1-5 rue Luigi Cherubini
FR-93212 La Plaine Saint Denis Cedex
Information
Tel. :+33 1 71 92 06 52 Fax : +33 1 71 92 07 97
www.arcelormittal.com/stainlesseurope Our recommendation
To avoid this type of corrosion, choose the appropriate grade in regard to the acid medium used.
We note the favourable impact of chromium and molybdenum which reinforce the existing passive film but also the combined effect of noble alloys (nickel, molybdenum and copper) which slow down the dissolution of the material when the stability of its passive layer is broken.
Figure 7: Critical current «icrit» at the peak maximum in H2SO4 2M de-aerated at 23°C Transpassivity Passivity Pre-passivity Up Activity Immunity Current density (I) M B Cathodic curve Anodic curve 100 10 1 0.1 0.01 K09 K03 K30 K41 K39 K45 K44 16-4 Mn 18-9E 17-4Mn 18-11ML © M ar ch 2010, Ar celorMitt al - S tainles s E ur ope. F iche C or rosion.eng. While ev er yc ar e has been t ak en t o ensur
e that the inf
ormation c
ont
ained in this public
ation is as ac cur at e as pos sible, Ar celorMitt al - S tainles s E ur ope, in c
ommon with all Ar
celorMitt al Gr oup c ompanies, c anno t guar ant ee that it is c omple te nor that it is fr ee fr om er ror . K AR A™ is a tr ademark of Ar celorMitt al - S tainles s E ur ope, r egis ter ed in numer ous c
ountries. Design and c
onc eption: www .agenc embc om.c om Potential U UP = Pitting potential
In a low oxidising medium, the cathodic curve (M) cuts the anodic curve below the pitting potential: metal remains intact.
In a strong oxidising medium, the cathodic curve (B) cuts the anodic curve above the pitting potential: pits appear on the surface of metal.
