Is Stainless Steel Really Stainless?

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The rust resistance characteristics of this type of metal depend on the high percentage of chromium in its composition. When it is in enough amount, it contributes to the formation of a layer of oxide that prevents the spread of rust. This feature makes it similar to aluminum. An oxide layer with anticorrosion resistance is also formed on its surface. But chrome, unlike aluminum, is many times thinner. To prevent the spread of rust, it should evenly cover the steel surface. The main cause of rust on this type of metal is the lack of chromium. If the alloying element is not enough, an oxide layer of insufficient thickness forms on the surface. A common cause of rust formation is its contact with ordinary carbon or low alloy steel. And it has nothing to do with the ERW steel tube.

Also, corrosion often occurs during welding. For example, the use of cored wire leads to the deposition of free iron molecules on the metal surface. Entering a chemical response to oxygen, in contact with air, they quickly oxidize. As a result, pitting rust occurs. To prevent this, after welding, the surface of this type of metal must be cleaned with chemical solutions or abrasively.

In the production and use of this type of metal, it is necessary to ensure its isolation from ordinary steel. When metal parts are cut, for example, for tables, shelving, lifting structures, the smallest particles of iron turn into dust. Settling on the surface of stainless steel, it also becomes a hotbed of rust. Therefore, one should grind, cut, clean iron parts away from stainless steel. So, what are the main forms of rust?

Galvanic

It happens due to direct contact of metals of various types in a medium with good conductivity. An example of the latter is seawater. When designing stainless steel structures, it is necessary to consider its contact with other metal elements in a specific environment. To avoid its formation, one should cover the metals with protective coatings or eliminate the electrolyte.

Crevice

It happens at the junction of individual structural elements and fastening joints, especially if they are operated in seawater. Chlorine destroys the oxide layer, and a lack of oxygen slows down its recovery. The constant flow of seawater causes the spread of rust. The mechanism of formation of crevice rust is as follows.

  • The accumulation of aggressive ions in the gap, the displacement of oxygen.
  • The appearance of the anode in the gap. And the material outside the gap plays the role of the cathode in this case.

To avoid its formation, one should take care of the design of construction or let the air come in.

Pitting

It occurs under the same conditions as galvanic. A damaged stainless-steel spot becomes an anode. Passivated sections are cathodes. Under such conditions, the anode is more likely to dissolve. As a result, pitting rust develops. This type of metal, as well as alloys based on aluminum and nickel, is most affected by pitting rust. Unlike ordinary steel, which often suffers from general surface corrosion, such materials in most cases are covered precisely by pitting – minor defects. Local destruction of the passive layer occurs in such situations: scratching, mechanical damage, a local change in steel composition, point effect of chlorine ions, sulfur, halides, temperature increase.

To avoid its formation, one should pay attention to the metal with a high content of chromium, molybdenum, and nitrogen.

Intergranular

It occurs when a metal lattice is deformed. Such an effect occurs, for example, when strong heating. Therefore, intergranular rust is most often formed during welding. Rusting begins if while heating, chromium carbamide appears.

To avoid it, one should choose grades with a low level of carbon.

Stress

It occurs when mechanical stresses, in combination with high temperatures, lead to the formation of small cracks.

To avoid it, one should choose steel with a nickel content of more than 30%.

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