What is Passivation of Stainless Steel Tubing?
What is Passivation?
Passivation is a process that removes free iron particles from the surface of stainless steel tubes to improve corrosion resistance. Free iron residing on the surface of a tube will have a propensity to convert into iron oxide (rust) overtime under certain conditions. What starts as a microscopic spec of iron oxide can propagate into further degradation of the tube surface in the same way that an untreated scratch in the paint of an automobile door will eventually lead to a growing rusty surface surrounding the original scratch.
Why is Passivation Needed?
But if the stainless steel is “stainless”---why is passivation needed? The magic of stainless steel is an atomic level chromium oxide boundary layer on the surface of the tube. This invisible layer acts as a shield to prevent any oxidation (rust) of the tube. It is self-healing under most circumstances. However, foreign material on the surface like free iron or machining residues can prevent the chromium oxide layer from re-forming, or weaken it such that a break in the layer can permit rust to start. Stainless steel is definitely “stainless” not “stain-proof”. Treatments like passivation enhance the stainless properties of any tubular parts.
Typically, passivation is one of the final processing steps in manufacturing a stainless steel tube part, whether a simple cutoff or a complex assembly. The reason why passivation should occur at the end of the manufacturing process is that along the manufacturing process, operations that alter the surface of the tube (such as any form of machining, grinding, cutting) can introduce free iron particles that remain on the surface. The goal of passivation is to remove these particles, rendering the surface fully “passive” when manufacturing is complete.
The most common ASTM passivation standard for tubing is A967 which specifies treatment in either citric or nitric acid at specified temperatures, acid ratios, and durations to achieve passive surfaces by dissolving and removing the free iron from the surface. Citric acid is used for the vast majority of treatments because it is more environmentally friendly. Typically nitric acid is used only for legacy tube components---parts in production for decades whose processes cannot be changed without expensive re-validation. Passivation does not leave a residue or change the color of the part, nor does it result in any measurable change in dimensions. Other passivation specifications commonly utilized include ASTM A380 or ASTM B912.
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