What are AP, MP, BA, and EP-grade stainless steel pipe

In addition to 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. In order to further improve the characteristics of pipeline materials, there are generally four methods for treating the inner and outer surfaces of the pipeline: pickling passivation (AP), mechanical polishing (MP), bright annealing (BA), and electrolytic polishing (EP) treatment processes. Both experimental data and practice have proved that after surface treatment, the roughness of the pipeline material is reduced and the surface adsorption is weakened. So what is the difference between AP, MP, BP, and EP grade stainless steel pipes?

First, AP (Acid Pickling) pickling treatment
The corrosion resistance of stainless steel mainly comes from the reaction of the chromium element contained in it with oxygen to form a dense protective layer on its surface. The role of pickling is to allow it to react in advance to improve the corrosion resistance of stainless steel pipes. The pickling process will remove the oil attached to the surface of the pipe and other substances contaminated during production. The inner and outer surfaces of the pipeline are relatively rough and prone to suspended impurities.

Second, MP (Mechanical polishing) Mechanical polishing
The polishing wheel or polishing belt uses the abrasive in the polishing agent to polish the surface of the stainless steel pipe, so that the stainless steel pipe surface can achieve a smooth polishing effect. Its brightness and effect depend on the type of processing method. In addition, although mechanical polishing is more beautiful, it will also reduce corrosion resistance, so when used in a corrosive environment, it needs to be passivated again, and polishing materials often remain on the surface of the steel pipe.

Third, BA (Bright Annealing) Bright annealing
During the manufacturing process of steel pipes, the grease grains used for lubrication will also be deformed due to processing. In order to avoid grease residue in the steel pipe, argon gas is used in the furnace atmosphere at high temperatures during annealing. Argon gas is combined with carbon and oxygen on the surface of the steel pipe to burn and further clean the surface of the steel pipe 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 bright annealing. Using this method to brighten the surface can ensure that the steel pipe is fully clean and free of any external pollution. However, the brightness of this surface will feel matte compared with other polishing methods such as mechanical, chemical, and electrolytic. Of course, its effect is also related to the content of argon and the number of heating times.

Fourth, EP (Electro Polishing) Electrolytic polishing
Electrolytic polishing is a method of anodic treatment. By using the principle of electrochemistry, the voltage, current, acid composition, and polishing time are appropriately adjusted. Not only can the surface be bright, smooth, and clean, but it can also improve the corrosion resistance of the surface. Therefore, it is the best method for brightening the surface. Of course, its cost and technology are also improved accordingly.

However, because electrolytic polishing will highlight the original state of the steel pipe surface if there are serious scratches, holes, slag inclusions, precipitates, etc. on the steel pipe surface, electrolysis failure may occur. The difference between it and chemical polishing is that although it is also carried out in an acidic environment, the steel pipe surface will not have any intergranular corrosion phenomenon, and the corrosion resistance of the steel pipe can be achieved by controlling the thickness of the chromium oxide film on the surface.