Having already invested in salt spray equipment for their other finishing processes, this becomes a less expensive alternative for passivation testing as well. This consistency causes many manufacturers to use it for testing zinc electroplate and other coatings. Proper controls and test piece setup make salt spray testing highly reproducible. However, the salt spray chamber pumps a 5% salt fog solution into its atmosphere for the duration of the test, which rapidly accelerates corrosion. Like humidity testing, salt spray testing places parts in a controlled environment for a period of time. This test is universal to all types of stainless but necessitates purchasing, calibrating and maintaining a humidity chamber, which can be expensive. Parts that pass this test exhibit no signs of rust or staining at the surface. Test specimens are left in the chamber for a full 24 hours. High humidity testing involves placing parts in a sealed chamber, the atmosphere of which reaches and a temperature of 100˚F and humidity levels near 97%. In such cases, it is more appropriate to conduct a high humidity test. However, this test is not suitable for the detection of ferritic contaminate in 400 series stainless steels because the composition of 400 series stainless will lead to false positives. Rather than constrain the passivation process, these standards focus on the methods of testing to verify that passivation was successful. ASTM recognizes that there are many valid ways to passivate a stainless steel part. Once the contaminant is removed, the passivation layer again has a chance to reform and make the component inert to corrosive reactions.Įnsuring Properly Passivated Stainless PartsĪSTM A967 and ASTM A380 – two of the most widely recognized industry standards for passivation – list a number of methods that can be used to passivate a part however, they do not claim to be all-encompassing. Passivation, a cleaning operation, removes contaminant on or just below the surface of stainless steel parts. To ensure a stainless part continues to resist corrosion after manufacturing, the part must be passivated in post-processing. Free iron inhibits the formation of the passivation layer, leaving the base stainless steel material exposed to the environment. This residual iron – known as “free iron” – can become embedded in the surface of stainless parts on contact. This form of corrosion (commonly known as “pitting corrosion”) occurs when residual iron is left on tools, containers, and other equipment used by a manufacturer to process carbon steel parts.
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