The Corrosion Behavior of Four Commercial Steels in Reducing Atmospheres Containing HCl At 773–873 K

The corrosion of four Fe–Cr commercial steels with different chromium contents in a simulated waste-gasification atmosphere containing 0.5 vol. H₂, 0.5 vol. HCl, balance CO₂ has been investigated at 773 and 873 K. The same materials have also been tested in the same gas mixture free from HCl at both temperatures for comparison. The results show that the materials with low-chromium content (2.25CrMoV and NF616) undergo accelerated corrosion in the presence of HCl, while the stainless steel SS304 suffers very little corrosion. On the contrary a steel containing 12 wt. Cr (12CrMoV) corrodes rather rapidly at 773 K but quite slowly at 873 K. The beneficial effect of chromium on the corrosion resistance of the steels increases with the chromium content at both temperatures. The steels tested show corrosion rates generally decreasing with time, having kinetics which are approximately parabolic at 773 K but intermediate between parabolic and linear at 873 K. Only little or even no chlorine can be detected at the scale/metal interface at both temperatures for all materials corroded in HCl-containing atmospheres. The corrosion mechanism can be explained by the so-called active-oxidation model.     

Oxidation of a Three-Phase Cu–45Ni–30Cr Alloy at 700–800°C Under 1 atm O2

The oxidation of a ternary Cu–Ni–Cr alloy containing approximately 45 wt.% Ni and 30 wt.% Cr has been studied in 1 atm O₂ at 700–800°C. The alloy contains a mixture of three phases: the one with the largest copper and lowest chromium content forms the matrix, the one with an intermediate chromium content has a rather large volume fraction and forms large islands, while the phase richest in chromium forms isolated particles dispersed in the other two phases. At variance with another Cu–Ni–Cr ternary three-phase alloy containing only 20 wt.% Cr and 20 wt.% Ni, which formed complex scales containing mixtures of the oxides of the various components and double oxides, plus an irregular region composed of a mixture of alloy and oxides, the present alloy is able to form protective, external chromia scales. A similar result could be obtained with alloys containing about 20 wt.% Cr, but composed of either a single phase (Cu–60Ni–20Cr) or of a mixture of two phases (Cu–40Ni–20Cr). The need for a larger chromium content for producing chromia scales for three-phase as compared to two-phase Cu–Ni–Cr alloys is attributed to the limitations of the diffusion of the alloy components in the metal substrate imposed by their multiphase nature.