The Difference Between Pickling and Passivation of Stainless Steel

Pickling and passivation are different types of chemical metal finishing that provide corrosion protection on metal surfaces. Both processes remove excess (or exogenous) iron or oxidation from the surface of whatever metal undergoes the process. Pickling (removing an oxide layer) and passivation (applying an oxide layer) are different finishing processes.

Why it matters: In order to achieve the desired results, it’s important to understand that pickling is a preparatory process, while passivation is a finishing process.

What is Pickling?

Pickling, also known as chemical descaling, is the chemical cleaning of metallic surfaces to remove impurities, stains or weld scale. This is done with an acidic solution. Allegheny Surface Technology uses pickling solutions containing nitric and hydrofluoric acids to remove weld scale and underlying chromium-depleted layers of a metallic surface, in order to restore the corrosion resistance of stainless steel.

Pickling is one of several possible pretreatment steps in other processing, such as passivation or electropolishing. Pickling is also useful as a superior cleaning operation on welded structures. The most commonly referred standard for pickling stainless steel is ASTM A 380, Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment and Systems.

What is Passivation?

Passivation is the chemical process (also involving an acid) that removes “free iron” from the surface of stainless steel, enhancing corrosion resistance of the metal. Passivation forms a chromium rich surface, as an invisible, inert (or “passive”) oxide layer when exposed to an oxygen-containing environment, such as air. The most widely used standards for passivating stainless steel are ASTM A967, AMS 2700, and ASTM A380.

What is the difference between Pickling and Passivation?

Pickling	Passivation
Pickling is a cleaning process.	A cleaning step is required before passivation.
Removes surface iron contamination, weld scale and heat tint from the surface.	Removes free iron contamination from the surface.
Uses acids that take off the surface of the metal to treat impurities on an atomic level.	Uses mineral or organic acids like either nitric acid or citric acid which are not as aggressive as the acids used in pickling.
Leaves a greater change to the metal. Leave a uniform matte finish.	Does not change the appearance or properties of the metal.

Passivation and pickling are often used as companion processes to remove contaminants and oxidation. Together, they improve corrosion resistance, thereby extending both the life and overall value of your stainless steel components.

Allegheny Surface Technology has metal finishing experts who will assist you in deciding which choice is the best for your stainless steel finishing needs.