Stainless steel doesn’t rust because of a process that leaves a very thin coat of chromium or other corrosion resistant surface. The process is called passivation. Passivation is accomplished by depositing a layer of oxide on the surface of the stainless steel. This layer is only a few nanometers thick and protects the metal from any corrosion. The process may also include the removal of all free iron on the surface to prevent any weak spots for corrosion may develop. Citric passivation uses citric acid to remove free iron. It has been used successfully and is not harmful to the environment or to humans.
Passivation is a precision cleaning of sorts. It is used on very large surfaces to remove very small contaminants. It is also used on very small stainless steel parts to ensure that there will be no corrosion especially in a tough environment. Stents that are used in angiography are finished in this way. It is very important that the stainless steel used for medical purposes like in heart surgery or pins for broken bones can withstand moist and chemically tough environments like the interior of the human body.
Derouging is another similar process to passivation. Roughing is a type of corrosion that is found only on stainless steel. Metal that has experienced this type of corrosion can be corrected. Roughing may occur when there is a high incidence of iron on the surface of the metal. Or it may also be the result of temporary support welding jobs that leave a spot of low chromium when the supports are welded off. The correction must be made chemically. The chemicals will not harm the stainless steel, but do remove the rouge. After the chemical cleaning, the surfaces of the stainless steel are repassivated to protect them from further problems.
The whole process is actually quite safe and is mostly pH neutral and uses chemicals that do not cause harm. To passivate stainless steel is necessary for medical instruments or even for use inside the body. To have iron oxide buildup on stainless steel inside the body could become a big problem. But chemically treated metals inside the body could be a problem too. That, however, is not a problem with passivation. These methods and chemicals are completely safe. Citric passivation is a newer process and has proven even safer than traditional methods.
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