Sanitary tubing is typically made of high quality austenitic stainless steel such as 316L. The interior surfaces are usually electropolished when used in high-purity systems such as those found in the pharmaceutical production and biopharma industries. The systems assembled from the electropolished stainless steel are often made of tubing runs comprised of sections of tubing joined by orbital welds.
Welds are problematic in a high-purity system due to their tendency to corrode faster than the undisturbed electropolished surfaces of the tubing. When the welds corrode, an iron oxide-rich rouge forms on the surface. This iron oxide can become a contaminant in the product carried in the line by dissolution of metals into solution and/or rouge particles detaching from the surface and becoming particulate contamination.
The welds corrode more easily than the undisturbed metal due to heat-induced damage. The heating is necessary to create the weld. The weld itself and the surrounding heat-affected zone (HAZ) have areas of chromium depletion, which is one of the more important changes seen in the weld and HAZ. This chromium depletion is seen as a decrease in the chromium to iron ratio (Cr/Fe) as measured by XPS (ESCA). Chemical passivation attempts to correct the effects of depletion of chromium at the surface of the weld and HAZ. This correction of the chromium to iron ratio is accomplished by the selective removal of iron atoms from the crystalline metal alloy structure, not by the addition of more chromium. The iron is removed on an atomic basis in the upper molecular layers of the metal, with a typical enhancement of the chromium to iron ratio to a depth of 20 – 50 angstroms.
The Astro Pak UltraPass® process will be tested to demonstrate the efficacy of this proprietary chemistry with regard to correction of chromium depletion on welds in 316L sanitary tubing.
Testing
A supply of electropolished (EP) sanitary tubing was procured. The tubing was sectioned and welded by an orbital welder using inert purge gas during the welding operation. Post welding, the welds were cut into sections and coupons made that were approximately 0.75 inch
by 0.75 inch square. The coupons were numbered on the outside surface, and the inner surface used for XPS measurement of the Cr/Fe and the chromium oxide to iron oxide ratio (CrOX/FeOX).
The UltraPass® process is proprietary, and the chemistry and techniques involved will not be disclosed in this document. Forty coupons were passivated using the UltraPass® process, five coupons were passivated separately with nitric acid, with a control group of six coupons that were cleaned but not passivated to provide a baseline measurement for comparison.
After the passivation of the 45 coupons was complete, all 51 coupons were subjected to XPS/ESCA analysis to determine the chromium to iron ratio. The control group and the UltraPass® groups also had the chromium oxide to iron oxide ratios measured. The coupons were sent to Evans Analytical Group (EAG) for XPS/ESCA analysis on the weld areas to determine the Cr/Fe and CrOX/ FeOX ratios as an indirect measurement of improvement of corrosion resistance. The data generated at EAG was then analyzed by the Astro Pak Research & Development team to determine if any differences existed in the three groups – one group of six non-passivated controls, one group of 40 passivated by the UltraPass® process, and one group of five passivated with nitric acid as a “gold standard” for comparison. The results of the analysis are presented below.
Conclusions
All weld samples from the control group (non passivated post welding) failed to meet the ASME BP recommended minimum Cr/Fe of 1.3, establishing the need for passivation.
All samples from the UltraPass® group exceeded the ASME minimum Cr/Fe of 1.3 when measured by XPS / ESCA (as stated in the ASME BPE NONMANDATORY APPENDIX E PASSIVATION PROCEDURE QUALIFICATION Table E-1 Minimum Surface Requirements for Process Qualification Samples). The mean or average Cr/Fe value for the UltraPass® group was 2.87.
The data was analyzed using the independent t-test to determine if the group means were significantly different. The independent t-test (also called the two sample t-test, independent-samples t-test, or Student’s t-test) is an inferential statistical test that determines whether there is a statistically significant difference between the means in two unrelated groups. The differences in the group means for Cr/Fe and CrOX/FeOX for the two groups, control and UltraPass®, were highly significant, with values that were well below the generally accepted significance level (also called alpha) of 0.05. The calculated p value for the data which produced the Cr/Fe means was 5.30191E-12 and for the CrOX/FeOX means the p value was 7.41995E-16.
A comparison of the mean Cr/Fe produced by nitric acid per ASTM A-967 methodology and that produced by the UltraPass® formulation was also made. The mean value for the Cr/Fe for nitric acid passivated welds was 1.96 versus 2.87 for the UltraPass® passivated group. As the data from this experiment demonstrates, UltraPass® yielded considerably superior Cr/Fe results when compared to nitric acid passivation; however, to reach statistical significance at an alpha of 0.05, a larger sampling of nitric acid passivated coupons will be required.
The UltraPass® process resulted in effective passivation treatment that raised the Cr/Fe above the ASME BP recommended minimum, and raised the CrOX/FeOX as well. The data was statistically significant. The UltraPass® treatment resulted in surface chemistry of welds that was better than untreated samples in terms of the Cr/Fe & CrOX/FeOX as measured by XPS/ESCA.
The UltraPass® formulation also produced superior results when compared to the Cr/Fe produced by nitric acid. The passivated weld surfaces all exceeded the ASME BP recommended minimum Cr/Fe for stainless steel surfaces. As weld surfaces are typically more prone to corrosion when the depleted chromium condition induced by the heat of welding is in an uncorrected state, the UltraPass® chemical passivation process displays efficacy even in a worst-case scenario.