What Are The Reasons For Corrosion And Perforation Of 304 Stainless Steel Round Pipes

304 stainless steel circular pipes are popularly used in various fields, including chemical, petroleum, and nuclear power industries, thanks to their high corrosion resistance. The dense chromium oxide films formed on their surfaces enhance their durability. However, despite their exceptional corrosion resistance, these pipes can still undergo localized pitting corrosion. This type of corrosion typically begins at the material surface and progresses through two stages of nucleation and growth, causing rapid expansion in depth beneath the surface. Ultimately, this corrosion can lead to perforation of the pipe wall. To understand why 304 stainless steel round pipes corrode and pierce, it is essential to consider the mechanisms behind pitting corrosion.

1. Chloride ion destroys the oxide film

Stainless steel product pipes are known to have a high corrosion resistance, owing to the protective oxide film that forms on their surface. This film is activated and maintained by the presence of Cr and Ni in oxidizing media, which form a dense and stable oxide layer on the surface of the pipe. The oxide film effectively passivates the steel and reduces corrosion rates, ensuring that the pipe maintains its integrity over time. However, the activation of chloride ions can disrupt the oxide film, leading to localized corrosion and pitting of the steel surface. Therefore, understanding the role of chloride ions in the establishment and destruction of the oxide film is critical for improving the corrosion resistance of stainless steel pipes in harsh environments.

According to the adsorption theory, the root cause of the destruction of the oxide film by chloride ions is attributed to their strong tendency to attach to metals. The metals tend to preferentially adsorb these chloride ions, thereby pushing oxygen out of the surface of 304 stainless steel pipes. Since the passivation state of metals is determined by oxygen, the competition for adsorption points between chloride ions and oxygen arises. In certain cases, chloride ions can even replace the passive ions in adsorption, creating chlorides with the metal. However, the adsorption of chlorides on the metal surface is not stable, ultimately resulting in the formation of soluble substances that expedite corrosion processes.

The chloride ions expedite the downward corrosion process.

The presence of Cl in a medium can cause pitting corrosion in stainless steel, due to the ease with which Cl can harm the oxide film on the surface of welded stainless steel pipes. Cl penetrates this film and reacts with the steel surface, resulting in the formation of an anode, which causes pitting corrosion. These pits tend to develop in the direction of gravity and can excavate deeply, accelerating towards the depth once formed.

The presence of chloride ions in a solution can dissolve the oxide film on the surface of 304 stainless steel pipes. This occurs because chloride ions can attach themselves selectively onto the oxide film, causing oxygen atoms to be displaced. These displaced oxygen atoms may then combine with cations in the oxide film, forming soluble chlorides. When this happens, tiny pits with a pore size ranging from 20 to 30 μ m appear on the underlying metal, which are known as pit corrosion nuclei. These nuclei can grow into corrosion pores when the external anodic polarization conditions are met in the medium containing a certain amount of chloride ions. Natural corrosion can also occur when a medium containing chloride ions is exposed to oxygen or cationic oxidants, which can aid in the growth of corrosion nuclei into corrosion pores.

The damage to the local passivation film on stainless steel pipes results in the remaining protective film becoming the sole barrier against pitting corrosion. This makes the conditions for such corrosion even more potent and intense. With the electrochemical production processes in place, the electrode potential of activated state steel pipes significantly exceeds that of passive ones. This means that in the presence of an electrolyte solution, the thermodynamic conditions for electrochemical corrosion are active, with the activated state acting as the anode and the passive state as the cathode. Pitting corrosion on these pipes occurs as a result of the metal at the anodic corrosion point undergoing electrochemical reactions at a much faster rate than the rest of the surface area of the pipes. As a result, only a small portion is corroded, while the remaining part serves as a large cathode area. Ultimately, point corrosion is formed as the penetration effect becomes significant and the corrosion rate intensifies.