The first step in manufacturing stainless steel parts is the actual fabrication. All parts must be made to spec, and obviously must adhere to the guidelines of design and application.
But once that phase is complete, is the job done? The resounding answer is no. The next phase is passivation, and this is just as critical as the fabrication process. Passivation ensures the crucial corrosion-resistance of stainless steel parts, and it’s the difference between parts that perform as needed, and parts that fail. And if not done correctly, corrosion is inevitable.
First let’s look at why passivation is necessary. When stainless steel components have been made, they still have layers of grease, oil, and other substances on their surface. The surface of the components also have a naturally-occurring oxide film due to oxygen exposure. Then when it is in operation, it is subject to abrasion and corrosion, and can be weakened or damaged. Therefore, there is a twofold need: first, to remove the post-fabrication oxide layer, and second, to prevent corrosion during operation.
Accordingly, chemical passivation is a two-step process that addresses both of those needs. The first step of the process removes iron and/or iron compounds on the surface with a nitric acid solution—a strong mineral acid. The acid dissolves the iron and iron compounds without affecting the component’s surface. Next, an oxidizer converts chromium metal on the surface, creating a layer of chromium-oxide protection. The two-step procedure results in the removal of all impurities from the parts, while providing resistance to oxidation, corrosion, and chemical attack.
While there is no specific number that states how often parts should be passivated, the important thing to remember is that a company that specializes in the process can guide you according to your specific application. Likewise, passivation is a complex process, and doing it wrong can result in major issues going forward; therefore, it should be left to trained finishing specialists to process.
When done properly by those who specialize in passivating, parts will be treated to pass all testing requirements, including high humidity testing, salt spray, water immersion, and more. This is especially important for complex, highly-regulated industries, such as aerospace, medical, the military, and other areas where high-reliability and safety-critical parts are needed. Proper passivation adheres to all guidelines as outlined for type II, VI, and VIII listed in AMS-QQ-P-35 and Nitric 1-5 in ASTM A 967.
The results are stainless steel parts that are free of impurities and protected for future use. After all, quality fabrication means nothing if your parts are destined to fail.