Application of Acid Pickling Process on Steel Pipe Surface in Anti corrosion Construction

1. Preface

In the process of oil and gas resource development, the long-term stable operation of pipelines is very important. As offshore oil and gas resource development gradually moves towards the deep sea, the operating environment of pipelines is more harsh than shallow sea, and the design life index of pipelines has reached a higher level, with the highest index even reaching 50 years. This puts higher requirements on the anti-corrosion coating construction technology of pipelines.

The anti-corrosion coating construction of pipelines mainly consists of three parts, namely surface treatment, anti-corrosion coating, and finished pipe coating inspection. Among them, the surface treatment quality of steel pipes is the top priority in the entire anti-corrosion construction process. According to relevant research, its impact on coating performance can reach 50% to 70%. The surface treatment process commonly used in the domestic pipeline anti-corrosion industry is shot blasting rust removal, which can meet the surface treatment needs of steel pipes with light corrosion. However, when production pipes have long-term storage and severe corrosion problems, the surface of the treated steel pipes usually has high salt content and high dust pollution, which has a negative impact on the quality of coating construction.

2. The necessity of acid washing process

2.1 Effect of Salt Content on Coating Performance on Steel Pipe Surface

The presence of salt on the surface of steel pipes has three main effects: increasing the power for water to penetrate through the coating; Provide ions that conduct corrosive currents; Certain ions may catalyze corrosion reactions.

When salt is present on the surface of the steel pipe, the corrosion reaction is accelerated due to the catalytic effect of the salt. The salt content on the surface of steel pipes will have a certain impact on the corrosion resistance of coatings. The higher the salt content, the worse the corrosion resistance of coatings.

2.2 Influence of Dust Pollution on Coating Performance on Steel Pipe Surface

After shot blasting and rust removal treatment on the surface of the steel pipe, there will still be some small metal debris and dust pollutants left on the surface. The use of pressure-sensitive tape can detect the dust pollution level of the coating. The lower the level of dust pollutants, the better the surface cleaning effect.

The dust pollutants on the surface of the steel pipe will remain in the uneven anchor patterns on the surface of the steel pipe after shot blasting and rust removal. When epoxy powder is sprayed on the surface of the steel pipe during the anti-corrosion coating process, the dust pollutants are blocked between the epoxy powder and the base metal, affecting the bonding performance of the coating. During the high-temperature melting and leveling process of the epoxy powder on the surface of the steel pipe, the dust pollutants will affect the leveling process of the powder, causing pores under the powder coating after leveling. These pores, when infiltrated with a small amount of water vapor, will become a corrosion point under the overall anti-corrosion coating, causing pitting corrosion on the surface of the steel pipe.

2.3 Acid washing process principle

Due to the adverse effects of salt and dust pollution on the coating performance on the surface of steel pipes, strict control of these two indicators is required during project construction, usually requiring a salt content of less than 20mg/m ² and a dust pollution level of less than level 2. When the surface of the steel pipe is severely corroded and polluted, the shot blasting rust removal surface treatment process is usually difficult to meet the construction requirements of the project. The pickling process is an effective chemical treatment method proven by foreign industrial practice to improve the surface cleanliness of steel pipes. When the steel pipe comes into contact with the pickling solution, under specified conditions, it undergoes complex chemical and electrochemical reactions to form a phosphate crystalline or amorphous conversion film with a certain thickness (1-50 μ m) and four crystal waters on the metal surface. This film transforms the metal surface from a good conductor to a poor conductor, which can suppress the formation of micro batteries on the metal surface and effectively prevent metal corrosion. After acid washing, the steel pipes can be cleaned with high-pressure water to remove loose surface contaminants such as salt and metal debris. The cleanliness of the steel pipes is significantly improved compared to before acid washing.

3. Conclusion

During the construction process of pipeline anti-corrosion coating, the surface treatment quality of steel pipes significantly affects the coating performance. In order to improve the surface treatment quality of steel pipes, acid washing technology has been introduced into the coating construction process. Through practice, it has been proven that pickling process is an effective surface treatment process, and its effects are mainly reflected in the following two aspects:

(1) After acid washing treatment, the surface salt and dust pollution of the steel pipe is significantly reduced;

(2) After acid washing treatment, the cathodic stripping performance and hot water immersion resistance of the coating are significantly improved.