Stainless steel alloys receive their characteristic corrosion resistance due to the addition of chromium. Chromium oxidizes spontaneously when the alloy is formed in the presence of oxygen to produce a nanoscopically thin, surface oxide layer called the passive layer. All stainless steel passive layers contain chromium, iron, and oxygen, other elements may also be present in this passive layer depending on other alloy additions in the particular stainless steel grade. The presence of iron in the passive layer, specifically “free” iron embedded on the surface, is a detriment to the stainless steel’s corrosion resistance and must be removed. The chemical process to remove this free iron or other contamination from an already-existing passive layer is called passivation.
Passivation can be measured both qualitatively and quantitatively. Qualitative tests are non-destructive surface chemical reactions that determine if remnant free iron exists on the stainless steel surface. Quantitative tests use advance microscopic techniques (Auger Electron Spectroscopy, AES, and X-ray Photoelectron Spectroscopy, XPS) to determine the exact chemical composition and thickness of the passive layer. The primary measurement of surface chemistry received through AES or XPS is the ratio of total chromium to iron within the passive oxide layer, referred to as Cr/Fe. XPS is also able to measure the amount of oxidized chromium relative to oxidized iron, that ratio is referred to as the CrOX/FeOX.
Many industries and specifications consider the qualitative tests enough verification of an adequate passivation process, but some high-purity industries have developed criteria based on passive layer composition and thickness to qualify acceptable passivation methods. ASME’s standard on Bioprocessing Equipment (BPE) is an example of such from the biopharmaceutical industry. A compliant passive surface for BPE shall have a minimum Cr/Fe, measured through either AES or XPS, of 1.3 with a minimum oxide layer thickness of 15 Angstroms.
Passivation at this level of surface enhancement can be accomplished using a wide range of chemicals and process parameters and many service providers have formulated proprietary blends to meet these requirements. Astro Pak’s Ultra Pass® is one such blend and Astro Pak’s Technology Group has qualified this proprietary, citric acid-based, blend as compliant with the minimum requirements of the BPE Nonmandatory Appendix E.
Processes
Eight groups of five 316L coupons were subject to various passivation treatments. Table 1 below details the process conditions of the eight groups and four chemistries used. Three of the four chemistries are citric acid based: two proprietary blends developed by Astro Pak and a commercially available blend by the company Citrisurf. All groups were cleaned with a sodium hydroxide solution prior to passivation, one of the groups was not passivated to serve as an as-received Cr/Fe baseline. All liquid processes were immersion processes. The gel application process consisted of periodically scrubbing the application surface in 15-30 minute increments to facilitate the chemical reaction.
Table 1: Process Groups | |||
Group | Concentration [vol%] | Process Time [minutes] | Temperature [Celsius] |
As-Received (Cleaned Only) | N/A | N/A | N/A |
Nitric Acid | 30% | 30 | Ambient |
Ultra Pass® Short | Proprietary | 120 | 80 |
Ultra Pass® Long | Proprietary | 240 | 80 |
Gel-Pass Short | Proprietary | 120 | Ambient |
Gel-Pass Long | Proprietary | 240 | Ambient |
Commercial Blend Ambient | 7% | 60 | Ambient |
Commercial Blend Hot | 10% | 30 | 60 |
Results
After passivation, the five samples of each group were shipped to a third party laboratory for X-ray Photoelectron Spectroscopy (XPS, also referred to as ESCA) testing to determine the resultant passive layer concentration. XPS testing was conducted in accordance to the equipment settings and sample preparation technique defined in SEMI F60, Test Method For ESCA Evaluation Of Surface Composition Of Wetted Surfaces Of Passivated 316L Stainless Steel Components Figure 1 shows the average Cr/Fe and standard deviation from the five samples of each group while exact values are tabulated in Table 2.
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Table 2: Average composition of passive film |
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|
Group |
Chromium wt% |
Iron wt% |
Cr/Fe |
Std Dev, Cr/Fe |
|
As-Received (Cleaned Only) |
2.7 |
3.9 |
0.7 |
0.07 |
|
Nitric Acid |
2.0 |
1.0 |
2.0 |
0.41 |
|
Ultra Pass® Short |
3.7 |
1.7 |
2.5 |
0.81 |
|
Ultra Pass® Long |
4.3 |
1.8 |
2.4 |
0.20 |
|
Gel-Pass Short |
4.4 |
2.2 |
2.0 |
0.25 |
|
Gel-Pass Long |
2.8 |
1.3 |
2.3 |
0.81 |
|
Commercial Blend Ambient |
2.4 |
2.0 |
1.2 |
0.20 |
|
Commercial Blend Hot |
2.0 |
0.9 |
2.3 |
0.74 |
Discussion
Comparing the as-received sample group with the rest shows how necessary passivation is. The major change in surface chemistry after passivation is the reduction of iron content at the surface and not the increase in chromium. This illustrates well the practical application of the definition of passivation in the first paragraph of this paper; iron-based contamination is removed from these samples to restore or enhance the stainless steel’s corrosion resistance.
Aside from the ambient temperature commercial blend process, all passivation methods resulted in a surface chemistry compliant to the Nonmandatory Appendix E requirement. It is likely that the surfaces of the ambient commercial blend group had all free iron removed and would pass passivation any of the verification tests of ASTM A967, but its iron content is too high relative to chromium to be BPE-compliant. Compared to nitric acid, all high temperature or gelled citric acid process groups provided a higher Cr/Fe. Increased temperature or a form of agitation are required for citric acid passivation methods to result in greater chromium enrichment levels than nitric acid passivation provides. Further, all four Astro Pak proprietary citric acid methods resulted in an increased level of chromium at the surface compared to the as-received group. The nitric acid method removed both chromium and iron, with iron’s removal rate outweighing the chromium removal weight, while the four proprietary citric methods increased the amount of chromium present in the passive layer.
Conclusion
The Astro Pak Technology Group determined multiple compliant passivation methods for BPE’s chromium enrichment recommendation in its Nonmandatory Appendix E. All tested passivation methodologies aside from an ambient commercial blend process resulted in adequate Cr/Fe for use according to BPE with Astro Pak’s Ultra Pass® blend providing the highest level of enhancement.