Besides brand, material, and production process, the most important factor affecting the price of stainless steel pipes is the surface treatment method of the steel pipe. To further improve the material properties of pipelines, there are generally four methods for treating the inner and outer surfaces of pipelines: acid pickling (AP), mechanical polishing (MP), bright annealing (BA), and electrolytic polishing (EP) treatment processes. Experimental data and practice have proven that surface treatment reduces the roughness of pipeline materials and weakens surface adsorption. So what is the difference between AP, MP, BP, and EP grade stainless steel pipes?
AP (Acid Pickling) acid pickling treatment
The corrosion resistance of stainless steel mainly comes from the reaction between its chromium element and oxygen, which forms a dense protective layer on its surface. The purpose of acid washing is to initiate this reaction in advance, to improve the corrosion resistance of stainless steel pipes. During the acid washing process, oil stains and other substances adhered to the surface of the pipe during production will be removed. The inner and outer surfaces of the pipeline are rough and prone to suspended impurities adhering.
MP (Mechanical Polishing) mechanical polishing
The polishing wheel or polishing belt uses abrasives as the polishing agent to polish the surface of stainless steel pipes, promoting a smooth polishing effect on the surface. The brightness and impact depend on the type of processing method used. In addition, although mechanical polishing is more aesthetically pleasing, it also reduces corrosion resistance. Therefore, when used in corrosive environments, passivation treatment is required, and there are often polishing material residues on the surface of steel pipes.
Bright Annealing (BA)
During the manufacturing process of steel pipes, the oil grains used for lubrication may also deform due to processing. To avoid oil residue in the steel pipes, high-temperature argon gas is used in the furnace atmosphere during annealing. By combining argon gas with carbon-oxygen on the surface of the steel pipe and burning it, the surface of the steel pipe is further cleaned to produce a bright surface effect. This method of using pure argon gas for annealing, heating, and rapid cooling to brighten the surface is called brilliant annealing. However, using this method to brighten the surface can ensure the steel pipe is fully clean and free from any external pollution. Compared to other polishing methods such as mechanical, chemical, and electrolytic polishing, the brightness of this surface will have a matte feel. Of course, its effect is also related to the content of argon gas and the number of heating cycles.
EP (Electro Polishing) electrolytic polishing
Electrolytic polishing is a method that utilizes anodic treatment and the principles of electrochemistry to adjust voltage, current, acid composition, and polishing time appropriately. It can not only achieve a bright, smooth, and clean surface, but also improve the surface’s corrosion resistance. Therefore, it is the best method to brighten the surface, and of course, its cost and technology are also correspondingly increased.
However, because electrolytic polishing highlights the original state of the steel pipe surface, severe scratches, holes, slag inclusions, precipitates, etc. on the steel pipe surface may cause electrolytic failure. The difference between it and chemical polishing is that although it is also carried out in an acidic environment, there will be no grain boundary corrosion on the surface of the steel pipe. The corrosion resistance of the steel pipe can be achieved by controlling the thickness of the chromium oxide film layer on the surface.
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