by Lauri Jenkins, Victor Lusvardi, and Sharon Libert
Representative samples of corrosion resistant alloys and PFA were tested as to their relative resistance to corrosion and metalion extractables in a salt solution. Corrosion resistance is measured in terms of the critical temperature (Tc) for onset of crevice corrosion as defined by ASTM G48 1. The challenge solution for this test was 15 wt % NaCl. Results indicated an expected trend in the Tc for the metals (316L < AL6XN = I625 < C276). No corrosion was observed in the PFA sample even after an additional week of testing. Extractions were done in an aqueous 15 wt % NaCl solution adjusted to pH = 2.0 with HCl at 40°C. The results correlate with the known compositions of the metals, showing significant metal ion release even in the most corrosion resistant alloys. The PFA sample showed no detectable metal ion release relative to the metal samples. Extractions were conducted at 24, 72, and 168 hours.
* b/d signifies that the result was below the detection limit of the analytical method
** For a 2000L tank with a radius to height of 1:2, based on 24h extraction data
Stainless Steel Alloy Trace Extractable Metal Analysis: Tables 1 shows the results of the major ions extracted from the metal coupons and the PFA sample at 1 and 3 days of exposure to a solution of 15 wt% NaCl and pH of 2.0.
This extraction data demonstrates that even corrosion resistant stainless steel alloys demonstrate significant extractables. These results are supported by the trend shown in the results that the main metal ions extracted correlate with the metals composition of each alloy. The iron based alloys (316L and AL6XN) show iron as the highest concentration extractable ion. The nickel based alloys (I-625 and C- 276) show nickel as their primary extractable. This data shows that 316L has the highest extractables by far. The corrosion resistant alloys do show lower total extractables. Shown in Table 1 is a projection of daily metals contamination in a 2000L vessel, based on the 24 h results. This calculation assumes that the metal is in continuous contact with an environment similar to the test conditions. As indicated by these results, significant contamination exists from even the corrosion resistant metals which could be eliminated by use of fully fluorinated polymers as a lining or a material of construction in biopharma processing.
The biologics industry is investing significant capitol in switching from 316L SS to more costly corrosion resistant metal alloys such as I625, C276, and AL6XN. While this investment is reducing their corrosion and metal extractables problems, it is not eliminating the contamination. Stainless steel requires expensive passivation and electropolishing steps to meet validation. Electropolishing smooths the surface by reducing the height of asperities, but does not remove the crannies at the base of these peaks and can even create pits that lead to increase biofilm adhesion.2 In contrast, the metal contamination from the Teflon® PFA coupon is so low that it is analytically non-detectable. PFA also has greatly reduced biofilm adhesion relative to conventional electropolished 316L SS.2 These qualities should translate into reduced CIP operations, reduced maintenance costs, reduced time to validation, and improved product yields.