Passivation is a standard, but critical, process in the manufacturing of many stainless steel parts. When referring to passivation, much emphasis is placed on the process and the characteristics it imparts.
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However, a major and often overlooked factor is the damage that can occur if treated parts are mishandled during storage and transportation.
To understand the fragility of the coating, lets first look at what exactly passivation is and what it does. Although passivation occurs naturally on most type of stainless steel, the passivation process was developed as a way of making sure the process occurs uniformly and quickly.
Passivation is a chemical process with an oxidizing agent, such as nitric acid, that creates a layer of film only a few atoms thick. It is chemically inert, so it reduces the reactivity of the stainless steel. ASTM standards A380 and A967, as well as AMS-QQ-P-35 are most often referenced for passivation.
Re-machining of the Part will remove and damage the passivation surface
Once passivation is completed, additional machining will remove the thin layer created by the process. This removal occurs not just on the worked surface, but by flying chips of material, either from the cutting tool or the part itself which can damage other regions where they may land or ricochet.
Besides the mechanical removal of the passivation caused by re-machining parts, the cutting fluids, used to reduce heat damage, and flush away chips can weaken this thin layer. It is not uncommon for sulfides, among other chemicals, to be used in cutting fluids to improve the release of chips from the cutting surface of the end-mill or other cutters. These compounds can find weaknesses in the passivation layer, penetrate them, and create initiation sites for additional iron corrosion. Any of these occurrences will require the parts to be re-passivated.
Excessive Heat Treatments Can Impair the Films Ability to Prevent Corrosion
As a rule of thumb, passivated parts should not be heat treated. The heat treating process alters the crystalline grain structure of the metal, which affects characteristics such as hardness, strength, ductility or elasticity.
However, the exposure of a heated metal to any incidental oxygen will cause various metals on the surface to oxidize rapidly. This rapid oxidation usually creates a discoloration of the bulk material and a thicker oxide. The heat-formed oxide is both flaky and weakly attracted to the surface, allowing it to detach easily and expose additional material to corrosion. This flaky oxide is commonly referred to as ‘Scale’.
The rapid expansion and rearrangement of grains that occur within the heat treating process will disrupt the corrosion barrier created during any prior passivation. Once this barrier is disrupted, the passivation and scale must be removed, and the passivation process must be repeated.
Trying to passivate heat treated metals with scale will be unsuccessful as the passivation process, by itself, cannot penetrate the thicker layer of scale. So, this scale must be removed prior to final passivation, taking the original, thinner, passivation with it during the process.
Proper Handling to Prevent Contamination or Removal of the Passive Film.
Once a part is passivated, proper handling procedures must be followed to prevent the surface from being damaged, and allowing iron to be introduced back onto the surface.
It might seem that the stainless parts should be able to withstand normal handling procedures. However, there are two ways in which the passivation layer is removed during normal handling procedures. The parts can come into contact with corrosive agents, chemically damaging the chromium oxide or the parts are physically mishandled, mechanically removing the oxide layer.
After passivation, it is important to not let the parts come into contact with any chemicals that can damage or strip the oxide and re-expose sections of the native surface. This means protecting the parts from exposure to acids used in the passivation process, as well as exposure to weaker acids, such as vinegars or lemon and orange zests. Once a part has been passivated, it is best to handle it with care.
Storage of the passivated parts should be done in such a manner to not allow damage to occur. This can be accomplished by using a container that will not allow contact from foreign objects.
Conclusion
Although the passivation layer is a few atoms thick, it is a critical layer that gives stainless steel its defining corrosion resistance characteristic. In any oxygen rich environment, this material will begin to grow a thin layer of chromium oxide.
Any residual iron, or iron compound rich regions will negate this property by forming thick, weakly adhering iron oxides. These iron oxides will grow and degrade the surface of the part, causing it to fail prematurely. Stainless steel parts, once passivated, must be handled properly to reduce the risks of damaging the passivation layer.
Electro-Spec provides cost-effective, high-volume passivating services for high-reliability stainless steel components. For more information about passivation and its benefits, as well as Electro-Spec's passivation process, download our free eBook, An Overview of the Passivation Process.
