Under what circumstances is it necessary to perform acid washing and passivation on stainless steel?

The water pipelines in pharmaceutical factories need to be acid-washed and passivated~~~

Stainless steel surfaces with corrosion resistance requirements, hot-processed stainless steel surfaces, and so on.

This is just my personal opinion. First, let’s analyze it from the perspective of the function of pickling and passivation: pickling and passivating the surface of stainless steel serves to remove contaminants such as oil and iron particles from its surface, and to create a dense and uniform protective layer on that surface. In stainless steel processing, it is impossible to avoid damaging the surface protective film; therefore, I believe that all processed stainless steel needs to undergo acid washing and passivation in the final stage. In cases where it’s not possible to perform acid cleaning inside components such as stainless steel pipes, the only option is to try to protect them as much as possible.

Generally speaking, unless there are other requirements, stainless steel containers need to be acid-washed and passivated before leaving the factory; if there are surfaces that need to be polished, then those surfaces do not require acid-washing or passivation

Pickling is used to clean the surface of stainless steel, while passivation is intended to form a passivation film; therefore, except in cases where polishing is required, the surface of stainless steel needs to be pickled. If stainless steel is to be used for corrosion-resistant applications, it must undergo passivation, and the effectiveness of the formed passivation film is tested using the blue spot test.

1. Reasons for stainless steel’s corrosion resistance: The corrosion resistance of stainless steel is primarily due to a very thin, dense passivation film on its surface (about 1 um thick); this film isolates corrosive agents and serves as the fundamental barrier that protects stainless steel. Stainless steel passivation is dynamic; it should not be considered as a complete cessation of corrosion, but rather as the formation of a diffusional barrier that reduces the rate of anodic reactions. Typically, it tends to damage the membrane in the presence of reducing agents, while it can maintain or repair the membrane in the presence of oxidants. The corrosion resistance of stainless steel relies primarily on its surface passivation film; if this film is incomplete or defective, the stainless steel will still be susceptible to corrosion. 2. When stainless steel parts are exposed to air, an oxide film forms on them, but the protective properties of this film are not sufficient. 3. Surface damage occurs during the forming, assembly, welding, weld inspection (such as flaw detection and pressure testing), and marking of stainless steel equipment and components; this damage destroys their surface oxide layer, reducing the steel’s resistance to general corrosion as well as to local corrosion (including pitting and crevice corrosion), and may even lead to stress corrosion cracking. 4. Purpose of acid washing and passivation: In engineering, acid washing and passivation are typically carried out to maximize the corrosion resistance of stainless steel. Typically, a thorough cleaning is required first, including alkali cleaning and acid cleaning, followed by passivation with an oxidizing agent, in order to ensure the integrity and stability of the passivation film. One of the purposes of pickling is to create favorable conditions for passivation, ensuring the formation of a high-quality passivation film. Because acid washing removes an average layer of 10 micrometers thick from the surface of stainless steel, the chemical activity of the acid causes the dissolution rate to be higher in areas with defects compared to other parts of the surface; thus, acid washing helps to achieve a uniform balance across the entire surface, eliminating potential areas that are prone to corrosion. But more importantly, through pickling and passivation, iron and its oxides dissolve preferentially over chromium and its oxides, removing the chromium-poor layer and resulting in a concentration of chromium on the surface of the stainless steel. The composition and structure of this chromium-rich passivation film enhance the corrosion resistance of the stainless steel. Cleaning, pickling, and passivation of stainless steel surfaces not only maximize corrosion resistance but also prevent product contamination and enhance aesthetics. In summary, according to Clause 10.4.6 of GB150 \"Steel Pressure Vessels\", \"the surfaces of vessels manufactured from stainless steel and composite plates that require corrosion protection shall be pickled and passivated.\" This regulation applies to pressure vessels made of oil in the petrochemical industry; such equipment is in direct contact with corrosive substances, and pickling and passivation are necessary. However, if aesthetic cleanliness requirements do not necessitate acid washing and passivation for the stainless steel material itself, acid washing and passivation are still required for the stainless steel welds.

Passivation is required after processing and welding; we sometimes use it as well………………

Stainless steel inner surface with corrosion resistance requirements