"Now wait just a cotton pickin minute here folks!!!" Foghorn - leghorn the Rooster cartoon of my youth. ;) ;) :)
First of all, the terms "pickling" and "passivation" are often confused, but are distinct processes. It's really important to be clear regarding the differences between these 2 surface treatment processes with respect in the use of their applications to stainless steels.
1.) THE PASSIVE LAYER FOUND IN STAINLESS STEELS:
Let's see if my attempt to give some sense of order in the way one would properly "treat" the surfaces of stainless steels in order to maintain, or re-introduce unique, self healing (to a point) surface protection system - called the passive layer which in stainless steels happens to occur naturally whenever a "clean" surface is exposed to an environment that can provide enough oxygen to form the chromium-rich oxide surface layer.
This occurs automatically and instantaneously, provided there is sufficient oxygen available at the of the steel. The passive layer does however increase in thickness for some time after it's initial formation. The transparent passive layer quickly reforms if damaged, provide there is sufficient oxygen in the surrounding air or aerated contact conditions. In this way stainless steels can create, maintain and keep the corrosion resisting passive surface condition, even where mechanical damage (e.g. scratching or machining) occurs, thus having a naturally built in self-repairing corrosion protection system.
2.) CHROMIUM:
The chromium in stainless steels is primarily responsible for the self passivation mechanism. In contrast to carbon or low alloy steels, stainless steels must have a minimum chromium content of 10.5%, (by weight) and a maximum of 1.2% carbon. This is the definition of stainless steels given in EN 10088-1. The corrosion resistance of these chromium steels can be enhanced with the addition of other alloying elements such as nickel, moybdenum, nitrogen, and Titanium, or columbium (Niobium). This provides a wide range of steels with corrosion resistances over a wide range of service conditions as well as enhancing other useful properties such as formability, strength and heat resistance.
Stainless steels cannot be considered corrosion resistant under all service conditions. Depending on the type (composition) of the steel there wil be certain conditions where the the passive state is broken down and prevented from reforming. Here the surface becomes "active" which results in corrosion. On stainless steels ,active conditions can occur in small areas deprived of oxygen, such as at mechanical joints, tight corners, or at incomplete, or poorly finished welds. The result can be "localised" forms of crevice or pitting corrosin attack.
3.) COMPARISON OF DESCALING, PICKLING, PASSIVATION AND CLEANING:
The terms "descaling, pickling, and "passivation" are often confused, but are distinct processes as applied to stainless steels. It's important to be clear about the differences between these surface treatment processes as applied to stainless steels.
4.) DESCALING:
Descaling is the removal of a thick visible oxide scale from the surface. This oxide is usually dark gray. This process is done routinely in the manufacturing steel mill before the steel is delivered. Mill descaling is usually a two stage process, one to mechanically loosen the mill scale, the second to lift the loosened scale clear from the metal surface. The exposed metal surface is then usually pickled to remove the metal layer that was immediately beneath the scale. This stage of the process should be considered as a separate one, however. Although some slight scaling may occur in the high temperature heat affected zone of welds or during high temperature heat treatment processes on fabricated stainless steel parts, further descaling operation are not usually necessary.
5.) PICKLING:
Pickling is the removal of a thin layer of metal from the surface of the stainless steel. Mixtures of nitric and hydrofluoric acids are usually used for pickling stainless... Pickling is the process used to remove weld heat tinted layers from the surface of stainless fabrications, where the steel's surface chromium level has been reduced.
6.) PASSIVATION:
Passivation usually occurs naturally on the surfaces of stainless steels, but ti may sometimes be necessary to assist the process with oxidising acid treatments. Unlike pickling, no metal is removed from the surface acid assisted passivation. The quality and thickness of the passive layer is however quickly developed during acid passivation treatments. There may be circumstances when the pickling and passivation processes occur sequentially (not simultaneously), during acid treatments involving nitric acid. Nitric acid alone will only passivate stainless steel surfaces. It is not an effective acid for pickling stainless steels.
7.) CLEANING:
Acid treatments alone cannot be relied upon to remove oil, grease or inorganic contaminants that can also prevent the passive layer forming properly. Combinations of degreasing, cleaning, pickling and passivation treatments may be necessary to fully prepare machined or fabricated stainless steel surfaces for their intended service conditions.
If stainless parts are contaminated with grease or oil, then a cleaning operation prior to acid treatment should be carried out.
There's more to add however, I'm in the mindspace of taking a mental breather in the form of a nap for just a bit so, until the next time, I'll add some more to this then. ;) ;) :)
Respectfully,
Henry
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