Corrosion by molten mixtures of sodium carbonate and vanadium pentoxide
The problem of corrosion of steels by slags deposited during combustion of heavy petroleum fuels which contain sodium and vanadium becomes very severe when the temperature of the metal exceeds 550 to 600°C, thus placing undesirable limitations on both the manufacturers and users of heavy fuel oil fired boilers and turbines. To contribute to the understanding of this problem, this study was made to determine the mechanism of the reaction between iron and molten mixtures of sodium and vanadium oxides.
Tests were conducted on 1020 carbon steel in carbon dioxide and helium atmospheres at temperatures from 593°C to 927°C using a slag composed of 64 mole per cent vanadium pentoxide-36 mole per cent sodium carbonate, and at 927°C using sodium-vanadium slags with 100, 90, 50, 36, and 16 mole per cent sodium carbonate. Tests were also made with Hastelloy B, Hastelloy X. and 347 stainless steel. The metal specimens were coated with the slag, heated at a constant temperature in a combustion furnace for up to 36 hours, and cleaned and weighed to determine weight loss. The slag and corrosion products were analyzed by x-ray diffraction techniques.
The 64 mole per cent vanadium pentoxide slag was chosen for the most extensive study since it represents the primary eutectic of the sodium oxide-vanadium pentoxide system and favors the formation of the complex sodium vanadyl vanadate, Na₂O • V₂O₄ • 5V₂O₅, considered by many investigators to be the major cause of corrosion.
This work showed that the corrosion reaction results from the oxidation of iron by vanadium oxides, with the reaction proceeding as follows: (1) oxidation of iron to ferrous oxide with reduction of vanadium pentoxide to the tetroxide, (2) fluxing of the initial products of corrosion by the slag, and (3) simultaneous additional oxidation of iron and further oxidation of ferrous oxide to ferrosoferric and ferric oxide. At intermediate temperatures, 670°C, step (2) controls the corrosion rate for a portion of the reaction, as evidenced by two very distinct changes in slope of the weight loss-time curve. At higher temperatures, 927°C, no distinct changes in slope were observed.
Tests at various slag compositions show that the effect of carbon dioxide increases as the sodium carbonate concentration of the slag increases. Above a one to one mole ratio, increasing the vanadium pentoxide content of the slag increases the rate of corrosion. At 50 mole per cent vanadium pentoxide the rate of corrosion of 1020 carbon steel was approximately 17 milligrams per square inch, hour, while at 84 mole per cent vanadium pentoxide the rate was 65 milligrams per square inch, hour.
The tests conducted with alloy steels show that increasing the nickel content of the alloy from 12.5 to 61.0 per cent, increases the corrosion rate from 1.7 to 10.0 milligrams per square inch, hour.