How to Prevent Corrosion of Stainless Steel?

Definition and classification of corrosion resistance of stainless steel

Stainless steel is only corrosion-resistant steel, not completely stainless.

So far, there is no invention of steel that is noncorrosive under any conditions.

Therefore, specific stainless steel grades have specific application environments.

Corrosion resistance principle of stainless steel:

① The corrosion resistance of steel can be effectively improved by adding alloying elements Cr, Ni, etc. to increase the electrode potential of the base metal and reduce the number of micro batteries;

② The addition of alloy elements can make the steel obtain single-phase solid solution structure at room temperature, reduce the number of micro cells, and improve the corrosion resistance of steel;

③ The addition of alloying element chromium in the steel makes the surface of the steel form a Cr2O3 passivation film with dense structure, no corrosion medium and high resistance.

This passivation film has a compact and stable structure and a thickness of 1-6nm.

It is the protective film of the metal matrix, and the thickness and strength of the passive film will increase with the increase of the inscription content in the steel.

Classification and prevention of stainless steel corrosion

1. Pitting

Pitting corrosion (as shown in Fig. 1), is one of the common local corrosion of stainless steel.

Generally, the depth of the pitting is much greater than its diameter.

When there are some active anions (C1 -) in the medium, these anions are first adsorbed on some points on the metal surface, thus destroying the passive film on the stainless steel surface.

There will be defects, impurities and solutes in the steel.

Once the surface passivation film is damaged, the base metal is easily exposed at the surface defects, making it active, while the passivation film is passive, thus forming an active-passive corrosion cell.

Since the anode area is much smaller than the cathode area, the anode current density is large, and corrosion develops to the depth, the metal surface will soon be corroded into small holes.

Fig. 1 Pitting on SUS304 steel pipe used in brine

Ways to prevent pitting

① Select pitting-resistant materials, add aluminum to steel and increase chromium content, such as 316L steel;

② A reasonable heat treatment system is adopted to make the stainless steel matrix in a completely solid solution state;

③ Reduce the concentration of halogen ions in the solution and increase the PH value of the solution;

④ Stir the solution to avoid the local concentration of the solution and prevent impurities from adhering to the steel surface;

⑤ Improve the surface finish of stainless steel;

⑥ Reduce the temperature of the medium;

⑦ Adopt cathodic protection measures.

2. Crevice corrosion

A narrow gap is formed between metal and metal or between metal and nonmetal.

The movement of related substances in the gap is blocked, forming a concentration cell to produce local corrosion.

This corrosion is called crevice corrosion (as shown in Fig. 2), which often occurs at the connection of stainless steel equipment.

Fig. 2 Crevice corrosion at the bottom of the gasket in a flat heat exchanger used in brine

Ways to prevent crevice corrosion

① Materials resistant to crevice corrosion can be selected, including aluminum containing stainless steel and titanium-containing stainless steel;

② Improve the design scheme, try to avoid the design with gaps, or make the gaps open as much as possible.

Avoid the use of metal and non-metallic connectors as far as possible;

③ Increase the flow of medium liquid to prevent impurities and pollutants from depositing in the gap;

④ Increase PH value, reduce CI ion concentration and reduce crevice corrosion sensitivity.

3. Stress corrosion

Stress corrosion cracking (SCC) refers to metal and alloy cracking caused by corrosive medium and tensile stress at the same time.

Stress corrosion is characterized by the formation of corrosion mechanical cracks, which can not only develop along the grain boundary, but also pass through the grain.

The mechanical strength of the metal structure is greatly reduced due to the development of cracks inside the metal, which can suddenly damage the metal equipment in serious cases.

Ways to prevent stress corrosion

① Materials shall be selected correctly and materials sensitive to stress corrosion shall be avoided;

② Reasonable design to avoid excessive machining degree, large residual stress or stress concentration;

③ Pay attention to the use conditions to avoid corrosive media on the surface, especially to avoid local concentration of chloride ions.

4. Intergranular corrosion

Intergranular corrosion is a kind of local corrosion phenomenon that materials are corroded along the intergranular boundary, causing the loss of adhesion between grains.

Sometimes the appearance of parts damaged by this corrosion is still bright and intact, but due to the loss of adhesion between grains, the strength of the material has been basically lost, and in serious cases, the metal sound will be lost.

The cause of intergranular corrosion is generally believed to be the dilution of alloy elements at grain boundaries.

It means that the steel with sensitized temperature will precipitate chromium compounds at the grain boundary, and a chromium poor area will be formed along the grain boundary.

Under the action of the corrosion medium, the chromium-poor area along the grain boundary will be corroded first.

Intergranular corrosion may occur in austenitic, ferritic or duplex stainless steels.

The sensitization temperature range of intergranular corrosion of austenitic and duplex stainless steels is 450 ℃~850 ℃, and that of ferritic stainless steels is above 850 ℃.

Fig. 3 The intergranular corrosion around the weld of an AISI316 hook used in sulfuric acid solution

Ways to prevent intergranular corrosion

① The sensitivity of intergranular corrosion can be reduced with the increase of trace element content.

② The addition of stabilized elements titanium and saws can generate stable NbC and TiC due to the strong affinity between titanium and sharp and carbon, avoiding the combination of inscription and carbon, thus reducing the generation of intergranular chromium poor zone.

③ The presence of carbon, nitrogen, phosphorus, silicon and other elements is harmful to the resistance of materials to intergranular radiation, so the content of these elements should be reduced as much as possible.

④ During heat treatment, it is necessary to avoid staying in the sensitization temperature range for too long, and avoid precipitation of intergranular carbides and grain coarsening.

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