Passivation by definition of the ASTM A 967, is taking out the surface iron in a metal with an acid solution. In addition, passivation is also process of making something no longer chemically active, it is instead passive. The result of this is the passive surfaces of the material will become resistant to many reactions, even corrosion.
Types of Metals
Many metals like titanium and aluminum are for the most part able to pacify themselves. The oxygen in the air reacts to the atoms in the metals, creating a bed of oxide. For these certain metals, iron is left behind on the surfaces.
So, for these particular metals in order to pacify them, the iron must be removed from the surface. To do this, a simple acid solution is applied to the metal.
In addition, stainless metals as well as corrosion resilient metals can also generate rust as a result of the steel alloys they contain. In these types of metals which include, nickel and chromium, they must be left to react to the oxygen in the air in order of the oxide layer to form.
This layer will offer another level of protection from the rust forming on the corrosion resilient and stainless metal surfaces. Other parts of the alloys in these specific metals can linger behind on the upper layer of the repressed steel. This is an essential component in protecting the remaining steel from corrosion.
This process can be ruined through the different mechanical processes the steel can go through, and once this happens, the outer layer will be susceptible to rusting once again. This is where the passivation process comes in.
During this process in the ending step of making stainless steels, the surface iron would be removed rendering the reaming steel to be rust resistant. On the contrary, any exposure to chlorides, will cause chemical damage to the metal’s passive layer. Ocean salt water, bleach and salt are very damaging to the stainless steel.
The preferred acid to use for eliminating the iron in the metal is citric acid. This is due to the overwhelming safety precautions that citric acid poses as opposed to nitric acid. The atomic level of nitric acid kills the iron in the metal, but due to it’s acidity it also harms the other components in the alloy as well.
While getting rid of all of these elements that the nitric acid has removed as well, the waste product from it becomes toxic, causing danger to those who handle it. On the other hand, citric acid safely removes only the iron on the metal.
The only draw back is the metals contain lots of carbon or sulfur. These parts will remain on the surface of the metal posing as a sticky substance for sulfide and the carbon turns dark, mimicking the look of rust. In order to fix these problems, just remove the sulfur and the carbon with an alkaline mixture beforehand, and then you may remove the iron with the citric acid.
Blue colored or straw colored oxides can deposit themselves on stainless steel as a result of soldering or welding. A simple fix is to clean it with a scrubbing cleanser before using it.