How to deal with the oxidation of stainless steel seamless pipe?

Posted by zora li on April 17th, 2023

How to deal with the oxidation of stainless steel seamless pipe?

There are mechanical, chemical and electrochemical methods to remove scale from stainless steel pipes. Due to the complex scale composition of stainless steel pipes, it is not easy to remove the scale on the surface and make the surface highly clean and smooth. There are generally two steps in the descaling of stainless steel welded pipes, one is pretreatment, and the other is ash removal.

The pretreatment of the scale of the stainless steel welded pipe makes the scale loose, and it is easy to remove it by pickling. Preprocessing can be divided into the following methods:

1. Basic nitric acid melting treatment method.
The alkaline melt contains 87% hydroxide and 13% nitrate. The ratio of the two in the molten salt should be strictly controlled so that the molten salt has strong oxidizing power, low melting point and low viscosity. During the production process, only sodium nitrate containing halogen is analyzed not less than 8% (wt).

The treatment is carried out in a salt bath furnace at a temperature of 450-470°C for 5 minutes for ferritic stainless steel and 30 minutes for austenitic stainless steel. Likewise, iron oxide and spinel can also be oxidized by nitrates to loose ferric oxide, which is easily removed by pickling. Due to the high temperature, the formed oxide partly flakes off and sinks to the bottom of the bath furnace in the form of precipitate.

2. Alkaline nitric acid smelting pretreatment process: steam degreasing → preheating (150-250°C, time 20-30min) → molten salt treatment → water quenching → hot water washing.

3. Molten salt treatment is not suitable for steel pipes with weld gaps or curled edges. When the steel pipe is taken out of the molten salt furnace for water quenching, irritating alkali salt spray will be splashed, so the water quenching should adopt a deep bowl type anti-splash water quenching tank. When water quenching, first put the steel pipe basket into the water tank, stop above the water level, cover the water tank cover, and then lower the steel pipe basket into the water until it is submerged.

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zora li
Joined: August 16th, 2022
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