Passivation of Stainless Steel Tube
Stainless steel are designed to naturally self-passivate whenever a clean surface is exposed to an environment that can provide enough oxygen to form the chromium rich oxide surface layer, on which the corrosion resistance of these alloys depends. Naturally occurring conditions such air or aerated water will do this and so under many exposure conditions stainless steel will naturally self-passivate.
Principle of pickling and passivation of stainless steel
The corrosion resistance of stainless steel is mainly due to the fact that the surface is covered with an extremely thin (about 1nm) dense passivation film. This film isolates 1n corrosive media and is the basic barrier for stainless steel protection. Stainless steel passivation has dynamic characteristics and should not be regarded as a complete stop of corrosion, but rather forms a diffusion barrier that greatly reduces the anode reaction speed. Usually, membranes tend to be damaged in the presence of reducing agents (such as chloride ions), but can be maintained or repaired in the presence of oxidants (such as air).
Stainless steel workpieces will form an oxide film when placed in the air, but the protection of this film is not perfect enough. Usually, thorough cleaning, including alkali washing and pickling, and then passivation with oxidants are required to ensure the integrity and stability of the passivation film. One of the purposes of pickling is to create favorable conditions for passivation treatment and ensure the formation of a high-quality passivation film. Because an average of 10 μm thick layer on the surface of stainless steel is corroded through pickling, the chemical activity of the acid makes the dissolution rate of defective parts higher than other parts on the surface, so pickling can make the entire surface even and balanced, some of the original Hidden hazards that could easily cause corrosion are eliminated.
But more importantly, through pickling and passivation, iron and iron oxides are dissolved preferentially than chromium and chromium oxides, and the chromium-poor layer is removed, causing chromium to be enriched on the surface of stainless steel. This chromium-rich passivation film The potential can reach +1.0V (SCE), which is close to the potential of precious metals, improving the stability of anti-corrosion. Different passivation treatments will also affect the composition and structure of the film, thereby affecting the rust resistance. For example, through electrochemical modification treatment, the passivation film can have a multi-layer structure, forming CrO3 or Cr2O3 in the barrier layer, or forming a glassy state. The oxide film enables stainless steel to exert maximum corrosion resistance.
Pickling, passivation and removing iron contamination with nitric acid
Passivation treatments are sometimes specified, but it is important to consider whether this is strictly necessary or not. Stainless steel cannot be passivated unless the steel surface is clean and free from contamination and scale from welding operations.
Scale may need to be removed first by 'pickling' or mechanical abrasion and although the surface of freshly pickled stainless steel will normally be immediately passivated once the pickling acid has been washed off, it is important not to regard these two treatments as the same.
Pickling usually involves nitric acids / hydrofluoric acids mixtures, whereas, traditionally passivation has been done using only nitric acid. Nitric acid alone can be used to remove light surface iron contamination after which the acid facilitates the passivation of the cleaned steel surface.
Citric acid passivation as an alternative to nitric acid treatments
Citric acid treatments can also be considered as an alternative to nitric acid as both provide the oxidising conditions necessary for passivation. Citric acid is a less hazardous method and has environmental benefits in terms of 'NOx' fume emission and waste acid disposal. Solution strengths of 4-10% citric acid are specified for passivation treatments in ASTM A967.
Specifications for passivation treatments for stainless steel.
Traditionally the American standards have been used.
These include: -
ASTM A380 - Practice for Cleaning, Descaling and Passivating of Stainless Steel Parts, Equipment and Systems
ASTM A967 - Specification for Chemical Passivation Treatments for Stainless Steel Parts (based on US Defense Department standard QQ-P-35C)
In 1997 an alternative British Standard was published:
EN 2516 - Passivation of Corrosion Resisting Steels and Decontamination of Nickel Base Alloys
|
