stainless steel rusty
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Our project recently provided a batch of pipe and valve parts, located not far from the seaside.； Half a year later, the owner reported that it was rusty, but it was marked as 304ss when purchasing. Could it be that he bought a fake or for other reasons?
304's resistance to chloride ion corrosion is not very good. We have done experiments and found that it will rust after being soaked in 3.5% salt water for about a week.
Do not get water on it, otherwise it will corrode. If you look at it after a while, you will see yellow spots on it.
There may be something wrong with the material or it may be iron ion contamination during the production process.
The so-called stainless steel is actually relative stainless steel. This is not to say that it will never rust. Different stainless steels have different application ranges and working conditions. For example, almost all stainless steel is not resistant to chloride ion corrosion, except for a small amount of low-end stainless steel and high-end duplex stainless steel, such as 2205, 2605, etc., but the price is generally prohibitive, so it is recommended to use carbon steel lining.
Low-end chloride ion corrosion-resistant stainless steel such as 216, which is made with nitrogen added during smelting, can withstand very high concentrations when the pH value is not too low. However, the domestic smelting level restricts its application.
The seaside environment has high humidity and high Cl ion content, which may cause pitting corrosion of grade 300 stainless steel. In addition, as mentioned on the 4th floor, the contact between stainless steel and carbon steel is contaminated by carbon steel, causing damage to the passivation film, and the potential difference between dissimilar steels causes electrochemical corrosion.
Rusting of stainless steel is normal and is caused by environmental factors
Stainless steel is generally divided into ferritic stainless steel, austenitic stainless steel and pearlitic stainless steel. Austenitic stainless steel rusts under the action of chloride ions. The composition of seawater contains chloride ions. 304 is an austenitic stainless steel, so it will rust under the action of chlorine ions.
The fact that stainless steel does not rust is relative. In a certain environment and contact with certain media, its passivation film will be destroyed. It will rust or even be corroded.
304ss belongs to Austrian stainless steel, and chlorine ions corrode it greatly. The equipment is located at the seaside, and there are many chlorine ions in the seawater. It will inevitably be corroded if it is kept at the seaside for a long time.