Metal corrosion can be divided into uniform corrosion and localized corrosion. Uniform corrosion means that the corrosion products generated by the corrosive medium cannot form a tight protective layer on the metal surface. The corrosion process continues on the entire metal surface, causing the equipment wall thickness to become thinner and the load-bearing capacity to decrease. Local corrosion is concentrated on a certain part of the metal equipment. Although the amount of corrosion is not large, it will cause sudden damage to the equipment. This damage is difficult to estimate and often causes great economic losses and even accidents. Alkali embrittlement refers to the cracking of metal and alloy materials in alkaline solutions due to the combined action of tensile stress and corrosive media.

Corrosion mechanism

Carbon steel and low alloy steel are prone to corrosion and cracking in liquid alkali, nitrate, liquid ammonia and other media. The cracks are generally intergranular cracks. The segregation of carbon and nitrogen atoms or the precipitation of carbides at the grain boundaries become obstacles to dislocation movement, causing dislocations to corrode grooves or holes at the grain boundaries, causing stress concentration under the action of stress, and can easily develop into intergranular crack sources. Factors such as the adsorption of specific factors, the segregation of carbon and nitrogen atoms, and stress concentration can promote the expansion of metal crack sources. Specific ions of a specific medium can be adsorbed on the metal at the bottom of tiny cracks, reducing the energy required for metal cracking and making it easier for cracks to develop toward the crack front. Under the action of stress, the metal undergoes plastic deformation, and cracks expand until the metal ruptures.

Measures and protection

1, to increase the temperature of the alkali brittle fracture zone, select304Made of stainless steel or lined with furfuryl alcohol modified epoxy resin.

2, Reduce the use temperature of liquid alkali, use coil heating intermittently to maintain the temperature≤40℃.

3, Reduce the phenomenon of stress concentration, and perform certain heat treatment during the welding process of making storage tanks.