Why the stainless steel pipe to pickling
The corrosion resistance of the stainless steel pipe is good, the high temperature oxidation resistance is good, but also has the good low temperature performance and the mechanical and processing performance. Therefore, a large number of oil, chemical, nuclear engineering, power, marine, aerospace, medicine, light industry, textile and other fields. The main purpose of the use of stainless steel pipe is anti-corrosion rust. Corrosion resistance of stainless steel pipe is mainly rely on the surface passivation film, once the film is incomplete or defective, stainless steel pipe will still be corrosion.
In this regard, the project will generally be pickling passivation treatment, the corrosion resistance of stainless steel pipe to give full play to the potential. In stainless steel equipment and parts in forming, assembly, welding, weld inspection (such as inspection and pressure test) and construction of labeling process to bring oil stains, rust, dirt, low melting point metal non metal contamination, paint, welding slag and spatter, these substances will affect the surface quality of stainless steel equipment and parts, and destruction of the oxide film on the surface of stainless steel pipe, reducing the resistance to general corrosion and localized corrosion resistance performance of the steel pipe (including pitting, crevice corrosion), and even lead to stress corrosion cracking.
On the surface of stainless steel pipe cleaning, pickling and passivation processing, in addition to the maximum degree of corrosion resistance, but also to prevent product contamination and appearance.
In the GB150 – 1998 “steel pressure vessel”, “a table of containers made of stainless steel and composite steel plates that are resistant to corrosion shall be subjected to acid pickling and passivation”. This provision is for the use of pressure vessels in petrochemical industry, because these devices will be directly exposed to corrosive media, so in order to ensure corrosion resistance, it is necessary to propose acid passivation.
Other aspects, if not for corrosion purposes, as long as the cleaning and aesthetic requirements, stainless steel material does not necessarily need to acid passivation. However, the welding of stainless steel equipment should be done by acid pickling and passivation. In nuclear engineering, some chemical devices and other stringent requirements, in addition to pickling blunt outsider, to adopt high purity medium for final cleaning or fine mechanical, chemical and electrolytic polishing finishing processing.
What is the principle of stainless steel pipe’s pickling passivation?
Pickling is the removal of any high temperature scale and any adjacent low chromium layer of metal from the surface of stainless steel by chemical means.
Where the steel has been heated by welding, heat treatments or other means, to the point where a coloured oxide layer can be seen, there is a chromium depleted layer on the surface of the steel underneath the oxide layer. The lower chromium content gives lower corrosion resistance. To restore the best corrosion resistant performance, the damaged metal layer must be removed, exposing a fully alloyed stainless steel surface. Mechanical removal may leave abrasive or other particles embedded (interfering with corrosion performance) or may be impractical, so chemical means are usually employed.
Procedures incorporating pickling solutions of nitric (HNO3) and hydrofluoric (HF) acids remove the scale and the underlying chromium depleted layer and restore the corrosion resistance. Pickling solutions also remove contaminants such as ferrous and ferric oxide particles. Pickling solutions other than mixtures of nitric and hydrofluoric acids exist and can be used for specialised applications.
Pickling pastes, where the solution is mixed with an inert carrier, are commonly used to treat selected areas such as welds.
Pickling involves metal removal and a change or dulling in the visual brightness of the metal.
Electropolishing is a useful alternative to pickling. Metal removal is achieved, but usually results in a bright, smooth and more highly corrosion resistant finish.
Passivation is the treatment of the surface of stainless steels, often with acid solutions (or pastes), to remove contaminants and promote the formation of the passive film on a freshly created surface (e.g. through grinding, machining or mechanical damage).
Common passivation treatments include nitric acid (HNO3) solutions or pastes which will clean the steel surface of free iron contaminants. Care must be taken in selecting and using passivation treatments to ensure the selected treatment will target the contaminant. Passivation will also aid in the rapid development of the passive oxide film on the steel’s surface. Passivation does not usually result in a marked change in appearance of the steel surface.
Both pickling and passivation solutions can employ dangerous acids that can damage both the operator and the environment if not handled correctly. Stainless pickling acids are highly corrosive to carbon steel.
It is essential that all acids are thoroughly removed by rinsing the component after completing the process. Residual hydrofluoric acid will initiate pitting corrosion.
It may be advantageous to neutralise the acid with an alkali before the rinsing step.
The corrosion resistance of the stainless steel is affected by the roughness of the surface after polishing, with a marked decrease of the corrosion resistance as the surface roughness increases above a Ra value of about 0.5 micrometres. This roughly corresponds to the surface produced by grinding with 320 grit abrasives.
Either passivation or electropolishing can be used to improve the corrosion resistance of mechanically polished surfaces.
Pickling and passivation use strong acids, and normal precautions for safety should be followed. Consult Materials Safety Data Sheets and product packaging for detailed advice.
The corrosion resistance of stainless steel is mainly because the surface is covered with a thin layer (about Inm) of the dense passive film, the film will be corrosive medium isolation, stainless steel protection is the basic barrier. Stainless steel passivation is continuous dynamic. Stainless steel workpiece placed in the air will form an oxide film, but the film is not perfect protection. In general, it is necessary to make a thorough cleaning, including alkali washing and acid washing, and then oxidizing agent to ensure the integrity and stability of the passive film. One of the purposes of pickling is to create favorable conditions for passivation treatment, so as to ensure the formation of high-quality passivation film. Because the surface of stainless steel by pickling with an average of lOp m thick layer of the surface to be etched, the chemical activity of acid dissolved the defective parts of the rate higher than other bands on the surface, so the cleaning makes the entire surface evenly balanced, some of the original easy to cause corrosion hazards have been cleared.
But more importantly, by pickling passivation, can make iron and iron oxide than chromium and chromium oxide dissolved preferentially, eliminate the chromium depleted layer, resulting in chromium in stainless steel surface enrichment, the chromium passivation potential of up to +1.OV (SCE), close to the potential of the metal, the stability of high corrosion resistance the. Different passivation treatment will also affect the composition and structure of the membrane, thus affecting the stainless steel, such as modified by electrochemical treatment, the passivation film has a multilayer structure, forming a barrier layer in Cr03 or Cr203, or the formation of glassy oxide film, full stainless steel tube corrosion ability.
China Pipe Fittings Manufacturer www.ugsteelmill.com Exports Piping Fittings for flow control of oil, gas, water, steam, acid liquid, etc.
Source: Yaang Pipe Industry (www.ugsteelmill.com)