【Basic Knowledge 3】: What is low chromium? (Prize quiz)

It refers to the combination of carbon elements in stainless steel with chromium at the grain boundaries, resulting in the formation of Cr23C6 precipitates; this causes the chromium content at those grain boundaries to be lower than that on the surface or inside of the stainless steel. Since chromium is essential for ensuring the corrosion resistance of stainless steel, the chromium-deficient areas are the first to suffer from corrosion.

Chromium is the main cause of intergranular corrosion. The reason for intergranular corrosion in austenitic stainless steels is generally attributed to chromium depletion at the grain boundaries. Austenitic stainless steel possesses high corrosion resistance due to the high chromium content in the steel. However, if stainless steel is exposed to temperatures in the range of 450–850 degrees Celsius for an extended period, carbon in the steel will diffuse toward the austenite grain boundaries, where it combines with chromium to form chromium carbides. As a result, zones with a chromium content of less than 11.4% and a thickness of several dozen to hundreds of nanometers are formed on either side of these carbides. This chromium-deficient region prevents the intergranular areas from resisting the erosion by certain media. Therefore, such intergranular regions are highly sensitive to corrosive agents. Since it is difficult to avoid the temperature range of 450–850 degrees Celsius during the heating and cooling processes of the weld and heat-affected zone during welding, the intergranular regions of the welded joint metal can become chromium-deficient, leading to intergranular corrosion. In addition, other heat treatment or usage processes, such as when the temperature is within the sensitization range, can also lead to chromium depletion at the grain boundaries in austenitic stainless steels

1. The carbonide-saturated solid solution of austenitic stainless steel is unstable at high temperatures; carbides precipitate along the grain boundaries, and chromium separates from the carbides at those crystal boundaries. 2. The diffusion rate of chromium is much lower than that of carbon; therefore, only the chromium near the grain boundaries can be consumed, resulting in the formation of chromium-deficient regions. 3. The chromium content in the chromium-poor areas is much lower than what is required for passivation; their potential is lower than that of the grains and carbides, and a galvanic effect occurs when exposed to corrosive agents. 4. In this case, chromium carbide and the grains act as the cathode, while the chromium-deficient regions act as the anode; as a result, the grain boundaries are rapidly eroded, leading to intergranular corrosion. This is the so-called chromium deficiency theory, which is also the mechanism of intergranular corrosion.

Cr and C form Cr23C6, which precipitates out, resulting in chromium deficiency. The hazard is intergranular corrosion

1. The carbonide-saturated solid solution of austenitic stainless steel is unstable at high temperatures; carbides precipitate along the grain boundaries, and chromium separates from the carbides at those crystal boundaries. 2. The diffusion rate of chromium is much lower than that of carbon; therefore, only the chromium near the grain boundaries can be consumed, resulting in the formation of chromium-deficient regions. 3. The chromium content in the chromium-poor areas is much lower than what is required for passivation; their potential is lower than that of the grains and carbides, and a galvanic effect occurs when exposed to corrosive agents. 4. In this case, chromium carbide and the grains act as the cathode, while the chromium-deficient regions act as the anode; as a result, the grain boundaries are rapidly eroded, leading to intergranular corrosion. This is the so-called chromium deficiency theory, which is also the mechanism of intergranular corrosion.

Chromium is the main cause of intergranular corrosion. The reason for intergranular corrosion in austenitic stainless steels is generally attributed to chromium depletion at the grain boundaries. Austenitic stainless steel possesses high corrosion resistance due to the high chromium content in the steel. However, if stainless steel is exposed to temperatures in the range of 450–850 degrees Celsius for an extended period, carbon in the steel will diffuse toward the austenite grain boundaries, where it combines with chromium to form chromium carbides. As a result, zones with a chromium content of less than 11.4% and a thickness of several dozen to hundreds of nanometers are formed on either side of these carbides. This chromium-deficient region prevents the intergranular areas from resisting the erosion by certain media. Therefore, such intergranular regions are highly sensitive to corrosive agents. Since it is difficult to avoid the temperature range of 450–850 degrees Celsius during the heating and cooling processes of the weld and heat-affected zone during welding, the intergranular regions of the welded joint metal can become chromium-deficient, leading to intergranular corrosion. In addition, other heat treatment or usage processes, such as when the temperature is within the sensitization range, can also lead to chromium depletion at the grain boundaries in austenitic stainless steels