Congratulations! You did your due diligence and had components passivated to increase their resistance to corrosion significantly. Now what? As miraculous as passivated stainless steel’s ability to not corrode under normal circumstances appears, passivation is not a secret potion that magically imparts a lifetime of invulnerability.
What is Passivation?
As a quick re-cap, stainless steel with a minimum chromium content of 10.5-12% naturally creates an inert surface layer when exposed to oxygen. This layer of chromium oxide (Cr2O3) is only 0.1 to 0.3 nm (10-30Å), or about two to four molecular layers thick. It protects the “transition area” which has a high nickel content and is also roughly the same thickness. It is the nickel in this layer which protects the passive layer from pitting corrosion reactions with the higher concentrations of iron in the base layer, below. It is the iron in the stainless steel which, if exposed on the surface, will oxidize or rust.
Iron makes up the majority of the alloy. In addition to the minimum 10.5% chromium content, nickel makes up at least 8% and other metals may be added depending on the purpose of the steel. The ratio of iron to nickel to chromium varies in the different layers, described above, but on the surface a ratio of chromium to iron of 1 to 1 is considered sufficient to maintain basic passivation. However, if passivated using Astro Pak’s Ultra Pass® process, ratios of 2 to 1 can be regularly achieved, ensuring superior protection from corrosion depending upon the quality of the stainless steel.
A Chemical Eggshell
But, even with a thicker and more resilient passive layer, it is still fragile. It can be damaged or effectively destroyed by machining, chemicals or heat.
If the component is to be further processed by machining, the passive layer will be removed by the mechanical action of the tool that is performing the work. Not only is the chromium-rich area physically removed, but the tool will cut all the way down into the base metal, exposing the more prevalent iron. In addition, the friction of the tool on the part will scatter debris from the tool itself as well as from the part. The blade is probably made from carbide or high carbon steel for strength and durability. Such fragments can become embedded in the worked area, or the surrounding material. This leads to a situation where a highly ferritic piece of material is now penetrating the passivated steel creating a localized site for corrosion to take place and threaten the base alloy. Furthermore, depending upon the process being performed, lubricating chemicals or abrasive grits can also add to the damage.
Likewise, if the component is being welded to a larger structure, the heat alone will destroy the passivated layer as the area being welded is physically melted. And, as the metal of the part is fused with the metal of the structure as well as any welding material, the surviving passive layer in the immediate area will be damaged in the new structure.
Similarly, heat treating after passivating a component is not recommended. This process alters the crystalline grain structure of the metal, changing its characteristics. Beyond that, the heated metal will rapidly oxidize on its surface as it interacts with the oxygen present in the atmosphere. This heat-formed oxide, known as “scale” will easily flake off of the surface, exposing the metal below and potentially serving as a source of contamination if it is not remediated before the structure is put into service.
Restoring the Passive Layer
If the component has been processed mechanically, chemically or heat treated, then passivation must be performed again in order to regain its protective layer. In the case of scale, the oxide must be physically removed as the chemicals used in the process cannot penetrate it. Fortunately, areas discolored by heat can be passivated, and properly executed welds can be passivated to nearly the same levels as the surrounding structure after they have been mechanically and chemically prepared.
Passivation Protection While Handling and Storing
The main goal of proper handling and storage of passivated parts is to ensure that they are not exposed to situations or environments where free iron can become exposed or embedded in their surface. To that end, they need to be properly protected until they are put into service.
Physical mishandling can cause damage to the passive layer as parts move against each other. While occasional jostling or movement generally won’t put the parts at risk, sustained vibrations or the like can lead to the layer being abraded.
Similarly, exposure to acid, moisture or other corrosive chemicals can also effect the chromium oxide layer. Such chemical interactions destroy the passive layer, and perhaps the underlying material as well. At the very least, they expose the iron within the metal, enabling it to bind with oxygen to form rust.
Proper storage for passivated parts must be in a manner which protects them from exposure to these hazards. For example, an environmentally controlled, dry, stable area would be far superior to the parts being stored in an open-topped box located near a continually-operated machine near the ocean. Clearly, such an extreme example is an unlikely situation, but with a small amount of forethought and planning, one can take steps to ensure that the components are properly stored so that they are readily useable without requiring further corrosion remediation.