Performance of high-alloy austenitic stainless steels in the actual and simulated bleaching media of the paper industry

Abstract

N08904 stainless steel pipes in a blanching plant of a paper mill suffered corrosion after 4 months. Service conditions were: temperature 64°C, pH 2.4-3, ClO 2 (as residual chlorine) 42–162 ppm, chlorides 312–483 ppm. The effects of these parameters on the behaviour of N08904 stainless steel and S31254 stainless steel have been studied with the aim of determining the limits within which each material can be used. Potentiodynamic-polarization, galvanostatic and potentiostatic techniques were used, complemented by optical microscopy. In reducing, non-aggressive media, both steels show low passive currents, wide passive-potential ranges, secondary passivation peaks at about 1.05 V and evidence of oxygen evolution from 1.35 V onwards. Under service conditions, the addition of sodium chloride promotes an increase in the passive current and a decrease in the passivation-potential range of the N08904 stainless steel, as expected, i.e. the breakdown potentials of the passive films are always less positive than the secondary-passivation peak potential, except for [Cl −] < 0.005M . Galvanostatic experiments show that the N08904 stainless steel is unable to maintain the passive state, even at the lowest [Cl −]. Pitting corrosion is evident and crevice corrosion under the corrosion product becomes the predominant anodic process. The performance of S31254 stainless steel at the [Cl −] tested shows the beneficial effects of molybdenum additions, the passive-potential range comprises the secondary-passivation peak even at [Cl −] = 0.1M (12 times the service conditions) and the only impaired effect detected is higher anodic kinetics at oxygen evolution potentials. The enhancement of the anodic processes and the increased solubility of corrosion products with temperature make the environment too aggressive for N08904 stainless steel at 50°C, while S31254 stainless steel behaves well at temperatures as high as 65°C. At 80°C, pitting and crevice corrosion are evident in the latter alloy. At 225 ppm of residual chlorine, the S31254 stainless steel loses its passivity and pits. Species such as ClO 4 −, ClO 3 − and ClO −, in equilibrium with the dissolved ClO 2, may well be aggressive to the alloy. The Cr 2O 3 solubility is affected by pH and strong pH effects can be expected. Experimental results show that lowering the pH from 2.4 to 1.8 is enough for the S31254 stainless steel to fail in the bleaching media. On the other hand, at pH⩾ 7, the N08904 stainless steel resists that environment.