By Dallas Dean, Study Director, Chemistry
Stainless steel medical devices that are treated to prevent corrosion can be verified using a salt corrosion test.
Chemical passivation is often performed according to ASTM A976. This standard describes methods of passivation using either nitric acid or citric acid in order to create a non-reactive surface on stainless steel. Section 16 Practice C of this standard discusses utilizing ASTM B117 salt spray testing to determine if the passivation is uniform.
The salt corrosion test is intended to verify the uniformity of the passive surface layer on chemically treated stainless steel devices. The testing involves a specially designed salt fog chamber that subjects the test sample to a 5 percent salt fog in a sealed environment. If there are no visibly detected areas of corrosion then the sample passes the test.
An additional test option is the gravimetric analysis of corrosion loss according to ISO 9227. This testing can be performed concurrently with the visual test if needed and provides information about how much mass was lost due to corrosion during the test.
Both tests are performed by subjecting samples to a 5 percent sodium chloride fog in a sealed chamber. The pH is maintained within the range of 6.5 to 7.2 under saturated conditions and elevated temperature. Due to chamber size requirements representative coupons may be necessary to test large samples.