Let you quickly understand the factors affecting the corrosion resistance of stainless steel fasteners

Factors Affecting Corrosion Resistance of Stainless Steel Fasteners

Stainless steel is generally "not rusty" in the atmosphere. The so-called stainless steel is a relative concept, and its corrosion resistance is conditional. Under certain conditions, stainless steel will also corrode. At present, there is no stainless steel with stainless steel and corrosion resistance in any corrosive environment. The more serious corrosion of stainless steel fasteners is the local corrosion, which mainly includes stress corrosion cracking, pitting corrosion, intergranular corrosion, crevice corrosion and fatigue corrosion.

1. Impact of environmental media

Certain corrosion types usually occur under some specific environmental conditions, and pitting corrosion is easy to occur in media with special ions. Crevice corrosion may occur in the crevice where the solution is stagnant or in the surface of the shield, at the junction of metal or metal and non-metal, and at the contact with rivets, bolts, washers, valve seats, loose surface deposits and marine organisms; the necessary conditions for the occurrence of stress corrosion cracking are tensile stress (whether residual stress or applied stress, or both) and the existence of specific corrosion media.

The environment affects the corrosion resistance of stainless steel fasteners. Taking pitting corrosion as an example, the composition, concentration, pressure, temperature and PH value of the environmental medium have an impact on the corrosion resistance of stainless steel fasteners. Stainless steel is prone to pitting corrosion in media containing halogen anions CI-, Br-, and I. It is generally believed that pitting corrosion can only occur when halogen elements reach a certain concentration. The pitting potential of stainless steel fasteners is related to the concentration of halogen elements. If anions such as OH-and SO42-exist in the medium, pitting corrosion of stainless steel fasteners will play a corrosion inhibition effect, and the effect will decrease as follows: OH->NO3->AC->SO42->CIO4-. At the same time, the temperature, PH value and flow rate in the medium all affect the pitting corrosion of stainless steel fasteners; the temperature increases, the pitting potential of stainless steel fasteners decreases, and pitting corrosion is more likely to occur; PH>10 makes the pitting potential rise, and the effect is less than 10. Very small, the general medium flow rate increases, and the pitting tendency decreases. For stainless steel fasteners, the flow rate that is beneficial to reduce pitting corrosion is about 1 m/s. If the flow rate is too large, erosion corrosion will occur.

2. The influence of chemical composition

The reason why stainless steel fasteners are corrosion resistant in certain environments is related to their passivation properties. When the steel is in a passive state, a dense oxide film can be formed on the surface, which blocks the corrosion process and temporarily stops the corrosion.

Chromium is the most basic element in stainless steel, and it is also a necessary element to improve the stability of the passive film of steel. When the chromium content reaches 12%, the alloy can reach the ability of complete self passivation. The self-passivation ability of the alloy determines the corrosion resistance of stainless steel to a certain extent. Therefore, the chromium content in austenitic stainless steel cannot be less than 12%, and the chromium content of SUS304 in the market is now less than 12%.

Nickel is an element that improves the corrosion resistance of steel, and is more significant in non-oxidizing sulfuric acid. The addition of nickel to chromium stainless steel can improve its corrosion resistance in sulfuric acid, acetic acid, oxalic acid and neutral salts (especially sulfate).

Manganese also improves the corrosion resistance of chromium stainless steel in organic acids such as acetic acid, formic acid and glycolic acid, and is more effective than nickel.

Molybdenum can improve the passivation ability of stainless steel and expand the scope of its passivation medium, such as the use of hot sulfuric acid, dilute hydrochloric acid, phosphoric acid and organic acids. Molybdenum-containing stainless steel can form a molybdenum-containing passive film, which has high stability in many strong corrosive media, and it can also prevent the damage of chloride ions to the film.

Silicon can improve the corrosion resistance of steel in hydrochloric acid, sulfuric acid and high concentration nitric acid. Stainless steel is generally added with 2%-4% silicon, which can improve its corrosion resistance in the above media.

3. Impact of organizational structure

Stainless steel according to GB/T20878-2007 "stainless steel and heat-resistant steel grades and chemical composition" standard in 143 grades. There are 18 grades of ferritic stainless steel; 38 grades of martensitic stainless steel; 66 grades of austenitic stainless steel; 10 grades of precipitation hardening stainless steel; and 11 grades of austenitic-ferritic (duplex) stainless steel. In general, in the chromium content of the stainless steel, the best corrosion resistance of austenitic stainless steel, followed by ferritic stainless steel, martensitic stainless steel is poor.

3.1 austenitic stainless steel

Austenitic stainless steels have excellent overall corrosion resistance in many media, but are most sensitive to intergranular corrosion and stress corrosion.

An important factor of intergranular corrosion is the organizational structure of the material itself, that is, the chemical composition difference between the grain and grain boundary of the metal or alloy itself, the grain boundary structure, the solid solution characteristics of the elements, the precipitation process, the solid solution diffusion and other metal problems, resulting in electrochemical inhomogeneity, so that the metal has the tendency of intergranular corrosion.

3.2 ferritic stainless steel

Ferritic stainless steel also has a tendency to intergranular corrosion, but compared with carbon, chromium-nickel austenitic stainless steel, the general general ferritic stainless steel is more susceptible to sensitization, which has a higher tendency to intergranular corrosion. Ferritic stainless steel has good resistance to stress corrosion cracking in chloride medium, which is much stronger than austenitic stainless steel, but it is not absolutely not produced. Cracks often originate from intergranular corrosion and pitting corrosion.

3.3 Austenitic-Ferritic (Duplex) Stainless Steel

Austenitic-ferritic (duplex) stainless steel has the characteristics of both austenitic steel and ferritic steel. Generally speaking, the corrosion resistance of duplex stainless steel is roughly the same as that of high-chromium ferritic stainless steel or chromium-nickel austenitic stainless steel containing chromium and molybdenum, and is affected by the proportion of organizational structure, but compared with austenitic stainless steel, duplex stainless steel has higher intergranular corrosion resistance and stress corrosion cracking resistance.

3.4 martensitic stainless steel

The corrosion resistance of martensitic stainless steel is worse than that of austenitic and ferritic stainless steel. Its main advantage is that it can be strengthened by heat treatment. It is suitable for fasteners with high requirements on strength, hardness, wear resistance and certain corrosion resistance.

3.5 precipitation hardening stainless steel

Precipitation hardening stainless steel has the characteristics of high strength and good corrosion resistance, and its corrosion resistance is not only related to the composition, but also closely related to heat treatment. The precipitation of fine phase and aging reaction are harmful to corrosion resistance. The high strength characteristics of this type of stainless steel may also cause hydrogen embrittlement or stress corrosion when used. This must be noted.

In short, stainless steel fasteners in a variety of organizational structure, phase and chemical composition content, such as sulfide, δ-ferrite phase, σ-phase, α'-phase, precipitation hardening stainless steel precipitates, sensitized grain boundaries and welds may have an impact on the pitting corrosion resistance of stainless steel fasteners.

Reproduced: Golden Spider Fastener Web