Pickling and passivation of stainless steel castings

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What is pickling?

 

Pickling is the method of removing scale and rust on the steel surface with an acid solution. This is one way to clean metal surfaces. It is usually performed together with pre-passivation treatment.

 

Typically, parts are dipped in an aqueous solution such as sulfuric acid to remove oxide films on metal surfaces. It is the pretreatment or intermediate treatment of electroplating, enamel, rolling, and other processes.

 

What is passivation?

 

If the corrosion product produced by the metal due to the action of the medium has a dense structure and forms a film (usually invisible) that tightly covers the metal surface, the surface state of the metal will change, and the electrode potential of the metal will greatly increase. Leap in the forward direction and become a corrosion-resistant passive.

 

For example, when Fe→Fe2+, the standard potential is -0.44V, and it jumps to 0.5~1V after passivation, it shows the performance of corrosion resistance of noble metals. This film is called a passivation film.

 

Metal passivation can also be a spontaneous process (such as the formation of an insoluble compound, an oxide film, on the metal surface). In industry, passivating agents (mainly oxidizing agents) are used to passivate metals to form protective films.

 

Common examples: cold concentrated sulfuric acid, cold concentrated nitric acid, iron, and aluminum can all be passivated.

 

Why do stainless steel castings need pickling passivation?

 

The main and only purpose of pickling is to remove scale from the surface of stainless steel castings. Therefore, pickling of stainless steel castings is not necessary. Only required if the scale is present on the surface. For example, stainless steel castings annealed with protective gases such as pure hydrogen and ammonia decomposition products do not require pickling.

 

Pickling of stainless steel is usually associated with passivation. Stainless steel castings that require pickling are usually passivated, because no passivation film is formed on the surface after pickling, or the thickness of the passivation film is very thin and has no effect. A subsequent passivation process must be added to form and improve the passivation film on the stainless steel surface. This film is the key for stainless steel, not rust and corroding in general media.

 

The advantages of passivation are as follows:

 

  • Compared with the traditional physical sealing method, the passivation treatment has the characteristics of absolutely no increase in the thickness of the workpiece and no discoloration, which improves the precision and added value of the product and makes the operation more convenient.

 

  • Since the passivation process is carried out in a non-reactive state, the passivating agent can be added and used repeatedly, so the service life is longer and the cost is more economical.

 

  • Passivation promotes the formation of a passivation film with an oxygen molecular structure on the metal surface. The film is compact, stable in performance, and self-repairing in the air. The chemical film is more stable and corrosion-resistant.

 

Pickling passivation process

 

  1. Pickling: hanging stainless steel parts → chemical degreasing (conventional alkaline chemical degreasing or surfactant degreasing) → hot water washing → tap water washing → first pickling → tap water washing → second pickling → tap water washing → transfer to the next One process (such as chemical coloring → recycling → tap water washing → hardening treatment → tap water washing → sealing treatment → tap water washing → drying → finished product).

 

  1. Passivation: Before pickling and passivating stainless steel workpieces, if there is surface dirt, etc., it should be cleaned mechanically, and then degreased and degreased. If the pickling solution and passivation solution cannot remove grease, the presence of grease on the surface will affect the quality of the pickling passivation. Therefore, degreasing and degreasing cannot be omitted, and lye, emulsifiers, organic solvents, and steam can be used.

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