Modern ideas about pitting corrosion of corrosion-resistant steels and alloys

Abstract

Purpose. To analyse, systematize and generalize modern representations about a pitting corrosion the alloying steels and alloys in the chloride-contacting environments.Methods of researches. The analysis of literary data according to the work purpose, comparative analysis, method of expert evaluations.Results. In particular, it has been shown that metastable and stable pitings, mainly, originate and develop in the vicinity of inclusions, which are composed of steels and alloys. Their nature and size directly affect the compressive strength of these structural materials. In addition, metastable pitings significantly pour on the intensity of growth of stable due to the redistribution of anode currents between them. At the same time, that they are larger and higher their number on the surface of the steel or alloy, then lower the anode currents in stable sweating and the speed of their growth. It is proved that in pitting metal dissolves selectively. As a result, in their vicinity solid phase diffusion of the main components of the alloy occurs, which leads to enrichment of the surface of metastable pitings with chromium, and stable iron. Because of this, metastable pitings are repatriated, and stable grow as a result of defect formation. Taking into account the characteristic features of selective dissolution of metals in metastable and stable pitings. It is proposed to identify them according to the coefficient of selective dissolution of Cr from pitings. In particular, if it is less than one, then the steel or alloy pits up with the formation of stable pitings, and, if greater, then metastable. It is shown that the chemical composition of steels and alloys also determines their pyrotechnic resis­tance in chloride-containing media. In this case, the most significant effect belongs C r, Mo, N. The positive influence of Cr and Mo on the compressive strength of corrosion-resistant steels and alloys is associated with mixed oxide films, which these chemical elements form on their surface. However, there is evidence that phosphorus contributes to the segregation of Mo and Cr atoms into the oxide film, and thus contributes to the growth of the compressive strength of these structural materials in chloride-contacting environments. At the same time, Cr contributes to increasing the solubility of nitrogen in a solid solution of austenite, which leads to the formation of ammonia complex in pitings, which reduces the pH of the corrosive medium in the pitting, that is why they are repacivated.Scientific novelty. Metastable pittings originate and develop before they are repaciated, when the cathodic reaction near the surface of steels or alloys occurs during oxygen depolarization, and stable – with oxygen and hydrogen on their surface as a result of hydration, dissolved alloy components in corrosion products.Practical value. The enterprises which make the heat-exchanging equipment working in the chloride-contacting reverse waters can choose optimal alloyed steels and alloys for their pitting resistance and to predict their corrosive behavior.