11. APPENDIX
11.3 PASSIVATION
Pharmaceutical equipment and high purity water systems are designed so that product contact surfaces are not reactive, additive, or absorptive so the drug product is not adversely altered. High purity water systems are primarily composed of austenitic stainless steel (SS) materials due to their corrosion resistant and con-taminant free properties. Passivation is performed to maximize the metal’s corrosion resistance. The stain-less steel is sulfuric/nitric/hydroflouric acid pickled at the mill to remove manganese sulfide inclusions, scale, and other impurities or imperfections from the surface of the steel. As the steel is removed from the pickling bath, a thin oxide layer forms immediately over the surface. This oxide layer is what renders the stainless steel passive and non-reactive to corrosion. Any 300 series chromium steels containing 17% or more chromium that has been handled, welded, or worked should be passivated prior to service and suitably cleaned prior to passivation.
Passivation is the method used to fortify the steel surface by strong oxidizing chemicals such as nitric acid.
The acid depletes the steel surface of acid soluble species, leaving the highly reactive chromium on the surface in a compounded oxide form.
11.3.2 Advantages of Passivation
When SS systems are fabricated, the welding process destroys the existing passive film and compromises the metal’s ability to ward off the corrosive process. This is particularly applicable in those zones that are either heat affected or have had residues remain in contact with the metal surface for prolonged periods.
Passivating then provides a method to restore the integrity of the metals corrosion resistant surfaces that were disturbed. Passivation must be proceeded by a cleaning process.
11.3.3 The Chemical Process
Excessive electron depletion of the upper film and an inadequate supply of oxygen (molecular O2) will ensure the formation of surface corrosion products. When this occurs, the chromium (Crn+) separates from the sur-face and opens the way for oxidation of the iron (Fe) and nickel (Ni), lower in the metal lattice.
Establishing a passive surface or film on austenitic SS is essential to maximize the corrosion resistance that the metals offer. Passive surfaces on these metals occur naturally when exposed to an oxidizing environ-ment. Sources of oxygen include air, aerated water, and other oxidizing atmospheres. Formation of a sub-stantial uniform oxidized corrosion resistant surface or film is the result of passivation.
Besides natural occurring passivation, chemical and electro-chemical processes can be used to obtain an anodic oxide film. Nitric acid solution (HNO3), is an oxidizing acid (depletes electron from the metal surface) which erodes the metal. This initial reaction or oxidation resists further chemical reaction on the metal sur-face. Metals that have such a state are called “passive” and the phenomenon itself is called “passivity.”
The chromium oxide film thickness typically ranges from 0.5-5.0 nm, averaging 2.0-3.0 nm. The chrome to iron ratio measured in atomic percent within the chromium oxide should be at least one with ratios of two or more being optimal.
11.3.4 Passivation Procedures
Numerous procedures are available for passivating; they share the commonality of consisting of four main steps which are:
1) Wash (Solvent Degreasing)
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2) Water Rinse
3) Acid Wash (Passivation Step) 4) Final Water Rinse
Proper preparation of the metal surface to obtain a uniform non-defective passive film mandates the metal surface be completely clean and void of any organic or inorganic soils, free iron, metallic contaminants, or corrosive products.
The First Step (Degreasing) of the procedure is designed to remove dirt, dust, oil, and grease. A water-soluble detergent is used to accomplish this, or a solvent.
The Second Step (Water Rinse) is required to remove dissolved and freed soils and the detergent itself from the metal being cleaned.
The Third Step (Acid Wash) is to remove free iron, metallic residues, oxides, and other corrosion products from the surface of the metal. By removing these soils from the metal surface and providing an oxidizing atmosphere, the passive film is allowed to form and the passivation is accomplished. Inorganic acids are typically used in this step of the procedure.
The Fourth Step (Final Water Rinse) - The acidic solution is flushed and the system is rinsed until the quality of the effluent is equal to that of the influent.
The American Society for Testing and Materials, ASTM A 380-96, “Standard Recommended Practice for Cleaning and Descaling Stainless Steel Parts, Equipment and Systems,” is an excellent source of information about passivation. It includes cleaning and passivation procedures, chemical applications, methodology, and testing procedures. The standard is valuable in establishing specific passivation and other specialized clean-ing procedures.
Establishing an effective passivation procedure can be obtained by using the following guidelines:
• Start with an accepted or specified procedure. (See the chart on the next page.)
• Obtain weld coupons from the system or have weld coupons made for testing purposes.
• Perform specified procedure along with alternate procedures to offer a choice, meeting specific situa-tions, or requests.
• Confirm the effectiveness of the procedures tested with specified field and/or laboratory testing.
• This process for confirming the effectiveness of a specified procedure or qualifying alternative proce-dures should be included in the passivation documentation being submitted as part of the final validation package.
11.3.5 Passivation Chemical Alternatives
Nitric acid, a strong oxidizing acid, is the most common acid specified for passivation. Besides its ability to produce a free iron surface, the acid supplies the oxidizing atmosphere needed for passivation to occur.
Because nitric acid is a corrosive chemical, extreme care must be used with handling, storage, and use.
Federal Specification QQ-P-35C (1988) is an excellent reference for obtaining guidelines when using nitric acid on a variety of stainless steel alloys.
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Although nitric acid has traditionally been the preferred passivating acid, the trend in use of passivating solutions is to reduce chemical aggressiveness and to make safety, cost, and the environmental impact of the waste solution effluents a consideration.
Citric acid and ammonium citrate (ammoniated citric acid) are gaining popularity as alternatives to using nitric acid. The safety these chemicals offer the personnel and the work environment are desirable qualities. The ASTM Standard A 380 (1996) refers to these acids as cleaning acids, not passivating acids. This distinction has probably been made because the acids are not oxidizers as is nitric acid. The standard states that the citric acid-sodium nitrate treatment is the least hazardous for removal of free iron and other metallic contami-nation and light surface contamicontami-nation. To achieve a true oxidation chelating agents in conjunction with citric acid and ammonium citrate has recently been introduced to the pharmaceutical/biotech industry.
Phosphoric acid is a weak oxidizing acid sometimes specified in passivation procedures; however, there is no formal documentation referencing the use of phosphoric acid as a passivating acid.
Chelants, otherwise known as sequestering agents or co-ordination compounds, which include all the stan-dard water softening compounds such as Sodium tri-polyphosphate (STPP), Nitrilotriacetic acid (NTA), and Ethylene Diamine Tetra Acetic acid (EDTA) may be compounded into acid passivation solutions to enhance metal ion extraction.
Orbital welding in conjunction with the increased use of electropolished tubing decreases the aggressiveness required of the passivating acids during the initial passivation. Decreasing acid contact time, temperature, and/or concentration accommodates the quality of the welds and already passive surface of the electropolished stainless steel.
11.3.6 Chemical Application Methods
Passivation can be accomplished using a variety of applications. Among these are:
When detergent washing, agitation or impingement provides the best results. During the acid wash step, chemical contact is usually sufficient. Recirculation is the preferred application method for performing passi-vation procedures. Recirculating allows flow rate criteria, usually specified at 5 feet per second (1.5 m/sec), to be achieved. Meeting flow rate requirements of a procedure should not be confused with particle removal.
Many people assume when high flow rates are used that particle removal will be achieved. This is not true.
Particle removal is achieved by including the total linear feet of the system into the appropriate mathematical equation. A recirculating water system of 1000 feet (300 meters) with a consistent tube diameter would require as much as 25 hours of filtered recirculation time for total particle removal.
Circulation Recirculating through distribution systems
One Way Intermittent Flow Large non-recirculating Long one way pipe runs systems distribution
Spraying Tank interiors
Tank Immersion Numerous small parts Prefabricated tubing
Swabbing/Wiping Isolated Areas/Tank/Equipment Equipment that does not allow
Exteriors spraying or other applications
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11.3.7 Tests for Cleanliness and Passivity
There are several tests available to determine an acceptable level of cleanliness. Should confirmation of cleanliness be required prior to continuing with the passivation procedure, the water break free surface test, wipe test, or ultraviolet light testing are just a few of the tests that could be performed. These tests are for gross cleanliness inspections as stated in the ASTM Standard A 380 (1996).
Once the passivation procedure is completed, a test method should be used to confirm or establish confi-dence that the passivation procedure has been successful. One inexpensive method is the Ferroxyl Test for free iron as set forth in the A380 (1996). The test is used to detect surface iron contamination, i.e., iron salt residue from pickling, iron tool scratches on the stainless steel surface, iron deposits at weld areas, and iron oxides. The testing solution is applied to the surface being tested. A blue stain appearing within 15 seconds of application indicates presence of free iron.
Testing for a passive surface is usually accomplished by looking for traces of free iron on the metal surface.
The assumption is made that if there is no detectable free iron, the metal surface is clean enough for a uniform oxide film to develop. Another excellent source for specific testing methods is the Military Standard 753B (1985). Both Standards discuss specific tests for detecting free iron. They include Water Immersion/
Water Wetting and Drying Test, High Humidity Test, Copper Sulfate Test, and Ferroxyl/Potassium Ferricya-nide-Nitric Acid Test.
Direct testing for a passive surface can be accomplished by X-ray Photoelectron Spectroscopy (XPS) testing which is used to measure the oxidation states of elements found on the metal surface. Another direct, de-structive testing method is Auger Electron Spectroscopy (AES) which measures the elemental chrome/iron ratio on the metal surface and sub-surface with depth profiling. The direct testing methods for passivity supply detailed information about the oxide film itself rather than indirect observations. XPS or AES testing offers direct evidence as to whether the passivation procedure being used is effective or not. These methods of testing are more costly than the other above mentioned tests and are ideal for use with weld coupons to determine the effectiveness of the passivation procedure for the system.
11.3.8 Modified Passivation Procedures
A passivation procedure can be modified to deal with a variety of soils, surface finishes and weld area conditions. Adjusting contact times and solution’s temperature and concentration would be the simplest way to modify a specific procedure. Sometimes detergent wash or acid wash chemicals are changed or modified with additives to remove certain soils. For example, when removing rouge, solutions containing sodium hy-drosulfite can be substituted for the detergent wash step of the procedure. Citric and Phosphoric Acid also could be used as they do have some ability to remove light rouging. Another example would be the use of Hydrofluoric Acid, or more specifically, Ammonium Bifluoride to remove silica scale. The descaling step and associated rinse would necessitate additional steps being added to the standard procedure.
It is important when developing a passivation procedure, that laboratory testing is performed to determine the effectiveness of your procedure. Without preliminary laboratory testing, an educated guess would have to be made and the results may not prove satisfactory.
Below is a guide that can be used for passivating and de-rouging stainless steel components, piping, and equipment. The chart has some possible options for determination of the contamination and a course of action.
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11.3.9 Contamination Analysis
Method 1 Filtration of 1 liter sample through a 0.2-0.5 µm filter and inspect.
Method 2 Quantitative analysis of the specified metals and organic compounds with wet chemistry tech-niques or as available.
Method 3 SEM or Auger Electron microprobe/spectroscopy for analysis of surface chemistry and contami-nation.
11.3.10 Cleaning and Passivation Method
Method 1 Clean surface with aqueous cleaning solution, apply passivation paste to surface, rinse surface with DI water until traces of chemicals are removed.
Method 2 Circulate cleaning solutions through piping or vessels by circulation method. Circulate cleaning solutions as required by procedure. Circulate passivation solution as per recommended condi-tions. Rinse surfaces once through with DI water until conductivity of inlet and outlet fluids are within tolerances.
Method 3 Spray cleaning and passivation solutions onto surfaces of vessels, containers, and equipment as per recommended conditions. Rinse surfaces for minimum of 30 minutes per each rinse stage, and perform triple rinse.
Method 4 Soak components or equipment items in treating solutions or tanks as per recommended condi-tions. The minimum soak time per each solution is two hours. Process requires cleaning, passi-vation, and rinsing as a minimum. The cleaning system should include circulation, filtration, and heating.
Condition/Status Contamination Cleaning & System Procedure Analytical Method Passivation Chemistry
Method
New Component N/A 2,4 3 2
Electropolished
Component Newly Welded N/A 1,3,4 1,2,3 1,2
New System - Tubing N/A 2 2,3,4 2
Component/System Discolored 1 1,2,3,4 1,2,3,4,6 2
(Gold Color)
Component/System Discolored 1,2 2,3 4,5,6 3
(Brown, Red/Brown Color)
Component/System Discolored 2,3 2,3 4,5,6 3
(Black, Blue/Black Color)
Cleaning and Passivation
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11.3.11 System Chemistry
Chemistry 1 Nitric acid passivation is performed at Ambient Temperature for 30 to 60 minutes and at 50-60°C for 20 to 40 minutes.
Chemistry 2 Alkaline degreasing is performed with detergents (phosphates, sodium hydroxide, and po-tassium hydroxide), pH buffers, and surfactant. The process will remove organic films and particulate debris from the surface of the stainless steel. Utilize approximately 1.0-2.0%
detergent, 0.2-0.5% buffer and 0.01-0.2% surfactant.
Chemistry 3 Citric acid/chelant passivation is performed with chelants, reducing agents, surfactants, and pH buffers. These systems are proprietary processes and the exact chemistry and percent-ages are not available. The chelant systems are able to remove most metal contamination from the surface including iron, manganese, aluminum, and copper. The systems include 3.0-5.0% Citric acid and a variety of chelants, reducing agents, pH buffers, and surfactants.
Chemistry 4 Mineral acid cleaning and passivation can be performed for iron oxide removal or passiva-tion. Typical mineral acids include phosphoric, sulfuric or sulfamic acid. These acids can be utilized at 3.0-10.0% concentrations and at a variety of temperatures. Sulfuric acid is not typically used due to its highly hazardous nature.
Chemistry 5 Intensified acid/chelant systems are utilized for removal of high temperature iron oxide films, silica scales, and organic/carbon films. These systems are a citric based solution with addi-tional organic acids, strong reducing agents, and acid chelants. These systems can use fluorides for silica removal. After strong acid cleaning in a reducing environment, it is recom-mended that an oxidizing flush be used to ensure oxidation at the surface, removal of or-ganic films, and sanitization of the system.
Chemistry 6 Sodium Hydrosulfite, a strong reducing agent, typically used at 5% by weight at 120 to 160°F for two to four hours.
11.3.12 Procedures 11.3.12.1 Procedure 1
Clean surface of organic film and other debris.
a) Rinse surface with DI water.
b) Apply gelled acid onto surface at ambient temperature.
c) Brush passivating agent on surface very 15 minutes, maintain a wet surface.
d) After one hour minimum, brush surface with sodium bicarbonate solution until all reaction ceases.
e) Rinse surface with DI water until all traces of chemicals are removed.
11.3.12.2 Procedure 2
a) Fill system with DI water and perform leak test with circulation pump.
b) Circulate for a minimum of one to two hours with alkaline degrease stages and heat to 60-80°C with filtration.
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d) Circulate for a minimum of one to two hours with passivating acid solution and heat to 60 - 80°C.
e) Drain and rinse with DI water.
11.3.12.3 Procedure 3
a) Fill system with DI water and perform leak test with circulation pump.
b) Circulate for a minimum of two hours with alkaline degrease stages and heat to 60 - 80°C with filtration.
c) Drain and rinse with DI water.
d) Circulate for a minimum of eight hours with intensified passivating acid solution and heat to 60 - 80°C.
e) Drain and rinse with DI water.
f) Flush with oxidizing/sanitization solution.
g) Drain and rinse with DI water.
11.3.13 Rouging
Rouging is seen in many water systems, usually high temperature (80°C) distilled water and clean steam systems. Rouge is not limited to storage and distribution systems; it also can be found in distillation and clean steam generating equipment. The main constituent of the rouge film is ferric oxide, but it can contain iron, chromium, and nickel of different forms. From Auger Electron Spectroscopy, it has been found that the outer layer of a rouge film is carbon rich, and the underlying region is iron and oxygen rich, probably iron oxide.
Over time, the film uniformly distributes itself throughout the system. The exact mechanism of the rouge formation and proliferation is unknown. Because the phenomenon occurs in systems that offer the most corrosive environment, it is thought that low molecular weight ions of the stainless steel, such as iron, are drawn to the metal surface or are dissolved and uniformly re-deposited throughout the system. Others feel the rouge is an external contaminant probably colloidal in nature that once in the system, uniformly deposits itself.
Rouging would seem to be very site (facility) specific because of the variety in appearance and texture.
Rouge can be observed in a variety of colors including; orange, light-red, red, reddish-brown, purple, blue, gray, and black. It can be a very loose film, dust like in appearance and texture that can be readily wiped off to a tight pertinacious film that requires scraping with a sharp instrument to be removed. In addition to the diversification already discussed, rouge can be multi-layered exhibiting different colors and textures. Tradi-tionally the red rouges are most common in high purity high temperature water systems, while the blue/black rouges are typically found in clean steam systems.
Evidence of the migration of rouging in distribution systems can be demonstrated by monitoring a system over a period of time. Key places to look for rouging are still and clean steam generator discharge lines, tank water/vapor interface, pump heads, Teflon® diaphragms on diaphragm valves, interior surface of tank spray ball, and heat effected area of welds. Rouge deposition seems to have an affinity for Teflon® and would be one of the first places to look for signs of system rouging.
In some cases, the rouging appears as quickly as a month or two after system start up. In other cases, it is
In some cases, the rouging appears as quickly as a month or two after system start up. In other cases, it is