Corrosion types of stainless steel

Stainless steel is not easy to rusty steel, stainless steel in the main alloying element is Cr (chromium), only when the Cr content reaches a certain value, the steel corrosion resistance, stainless steel generally Cr content of at least 10.5%. The corrosion mechanism of stainless steel is the theory of passivation film, that is, its surface to form a layer of very thin and strong and stable stable Cr-rich passivation film to prevent oxygen atoms continue to infiltrate and continue to oxidize, so as to achieve the ability of anti-corrosion.

The pattern of passivation film on the surface of stainless steel

 

People think that “stainless steel is not rusty, rusty is not stainless steel.” In fact, this is a lack of understanding of stainless steel a one-sided view of the wrong, stainless steel under certain conditions will rust. If we can intuitively understand the various types of corrosion of stainless steel, the face of stainless steel corrosion can have a corresponding response to reduce the loss. The following describes the actual types of stainless steel common corrosion, a brief introduction to the corresponding anti-corrosion measures, and focus on stainless steel stress corrosion causes and prevention measures.

Corrosion damage from stainless steel is mostly local corrosion damage, the most common intergranular corrosion (9%), pitting (23%) and stress corrosion (49%). [1]

 

Corrosion types of stainless steel

Intergranular corrosion

 

Intergranular corrosion is due to impoverishment on the grain boundary, leading to preferential corrosion of the region, and thus the overall separation of the grain from the metal. Heat treatment 450 ℃ ~ 850 ℃ temperature range C and Cr easy to form carbon chromium compounds (Cr23C6), the grain boundary consumption of Cr can not be added from the grain in time, so that the grain boundary area of poor Cr, which is caused by intergranular corrosion The essential.

For example: a company synthetic ammonia plant furnace gas preheat tube due to intergranular corrosion caused by cracking, leakage occurred. The intergranular corrosion of the preheat tube is due to the precipitation of Cr23C6 in the grain boundary of the austenitic stainless steel, and the intergranular corrosion cracking occurs in the pipe under the action of Cl- in the tube.

Microstructure and microstructure of intergranular corrosion

 

In order to prevent intergranular corrosion of stainless steel, the general solution:

 

1, the solution annealing metal is uniformly heated to 1050 ℃ ~ 1060 ℃, and then rapid cooling;

2, adding stabilized elements Ti, Nb, etc .;

3, the choice of low-carbon stainless steel.

 

Pitting

 

Pitting is a very dangerous local corrosion, the occurrence of small holes and then the phenomenon of rapid corrosion, serious can lead to perforation. Pitting the main factors are:

1, Cl- effect, Cl – stainless steel passive film local damage, leading to the site of the first occurrence of corrosion;

2, the temperature impact, the higher the temperature, the faster corrosion;

3, the surface attached to the contaminants to prevent the flow of oxygen.

 

For example, the daily life of stainless steel (201 or 304 stainless steel majority) sinks often pitting phenomenon. If some acidic or salt substances in the sink did not get timely treatment, it will lead to pitting corrosion of stainless steel sink.

Pitting corrosion phenomenon of stainless steel pipe

 

The precautions are as follows:

 

1, to prevent Cl – attachment;

2, a reasonable surface treatment, the formation of stable passivation film;

3, select the resistance to Cl-corrosive materials (such as the addition of Mo 316L stainless steel).

 

Stress corrosion

 

Stress corrosion refers to the damage caused by the metal in the corrosive medium under the action of tensile stress. The stress corrosion of stainless steel is the most serious corrosive form in the failure of stainless steel. If there is a slight crack in the process of corrosion, the expansion rate is several orders of magnitude faster than other types of corrosion, causing catastrophic consequences. Chemical plants, nuclear power plants, boilers and other stainless steel materials have appeared stress corrosion cracking phenomenon.

 

For example, ① a pharmaceutical enterprise hydroxyl acetonitrile synthesis of glycine project synthesis section to the concentrated section of the 316L stainless steel pipe to the weld occurred a serious stress corrosion cracking. The stress corrosion cracking in the accident originated from the weld defect, the intergranular corrosion occurred improperly in the heat treatment control of the welding process, and the residual stress in the welding resulted in the formation of stress corrosion cracking on the basis of intergranular corrosion.

316L pipe fittings with stress corrosion cracking

 

(2) The reactor reactor core is 304NG stainless steel in the reactor core of the reactor, and the Irradiation Assisted Stress Corrosion Cracking (IASCC) phenomenon occurs in the high temperature and high pressure and strong irradiated water environment. Strict control of the nuclear plant service environment is one of the most effective ways to solve the problem.

Irradiation Assisted Stress Corrosion Cracking (IASCC)

 

The study of stress corrosion experiments is based on stress corrosion characteristics and experimental purposes under different conditions. According to the material, environment, stress state and experimental purposes, has developed a variety of stress corrosion test method. Experiments were carried out according to experimental site and environmental quality, laboratory experiment and laboratory accelerated experiment. The experiment was carried out according to the loading method, constant deformation experiment and constant strain rate tensile test. Samples of the same stress corrosion test are generally pided into three categories: smooth specimens, notched specimens and prefabricated crack specimens.

CT samples of cracks

 

The stress corrosion problem of stainless steel has been extensively studied, and many different mechanisms have been put forward to explain the stress corrosion phenomenon, but there is no universally accepted mechanism. Because stress corrosion is a process related to corrosion, the mechanism must be related to the anode and cathode reactions in the process of corrosion. Therefore, the stress corrosion mechanism is mainly pided into two categories: anodic dissolution and hydrogen cracking, and in these two mechanisms Based on the development of the surface membrane rupture theory, active channel theory, stress adsorption cracking theory, corrosion product wedge theory and occlusion battery theory.

Theoretical model of surface film rupture

 

Factors Affect Stress Corrosion There are three main factors: material, corrosion environment and tensile stress. The material factors include element content, heat treatment, microstructure and surface state. The corrosion environment includes medium type, temperature, potential and liquid flow rate. The tensile stress includes service stress, installation stress, residual stress and tensile rate.

 

Solution:

 

1, to reduce the stress concentration of high stress components;

2, to prevent the introduction of chloride ions;

3, to avoid stress processing;

4, heat treatment, to eliminate stress;

5, the use of resistant SCC ferrite stainless steel;

6, the use of high-Ni steel

 

In the new era of rapid progress in science and technology and continuous improvement of scientific research level, the research and development speed of new materials still can not keep up with the speed of the problem. In the practical engineering application of stainless steel corrosion problems emerge in an endless stream, although the current protective measures have achieved some success, but there are still serious shortcomings, a variety of means of protection can only play a role in a certain range. Reasonable selection, real-time detection is still the key to control corrosion.

 

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