Steam Corrosion Resistance of New 12% Cr Ferritic Boiler Steels

Abstract

A new 12%Cr steel integrating good creep properties, fabricability and corrosion resistance up to 650°C is being developed. For each step of this development, long term oxidation tests (about 6000h and 8000h) were made in pure water vapour in the temperature range 600-650°C. The laboratory and industrial heats were tested in comparison with two 12%Cr steels (X20CrMoV12-1), a 9%Cr steel (T91) and a fined grain austenitic stainless steel (TP347FG). Corrosion damage was measured using mass losses obtained after a reducing descaling process. Weight loss and metallographic results confirm the good corrosion resistance in steam of the new steel, VM12, and show either 2 different corrosion mechanisms or the same mechanism but with 2 different spreading rates: for the VM12, one X20 heat and TP347 steels, the time required for the whole surface of the samples being covered with corrosion products is definitely longer than for the other 9-12 % Cr.

Oxidation of X20 in Water Vapour: The Effect of Temperature and Oxygen Partial Pressure

“…The interface between the outer Fe-oxide and the inner Fe-Cr oxide coincided with the original surface of the sample [1,2,8]. The time to breakdown of the protective oxide depended on the Cr and Si content of the steel [3,10,15] as well as on the surface conditions (i.e. polishing the surface with 1 lm diamond prior to oxidation reduced the time to breakdown as compared with grinding with 1200-grit SiC paper) [2].…”

“…Over the last 15 years, efforts were made to explain the accelerating effect of water vapour on the oxidation kinetics. Oxidation experiments were carried out at temperatures between 550°C and 800°C in humidified mixtures of nitrogen and 1-2% oxygen [1,2], in humidified air [3][4][5][6][7], in humidified argon [8][9][10][11][12][13][14] and in pure water vapour [15]. The water vapour content of the gas in those reported experiments varied between 2% and 100%.…”

Comparison between Current Methods for Measurement of High Temperature Corrosion of Boiler Steels in Steam Environments

“…1a presents the mass losses, classified by decreasing values. From our previous studies [6,7], the tested steels can be classified in 2 families:…”

“…To assess the oxidation kinetics, discontinuous measurements were used [4]. Taking into account the results of our previous studies [5,6,7], a maximum test duration of 32 weeks (5,376h) was chosen. Four intermediate samplings were carried out after 2, 4, 8, 16 weeks of test.…”