CORROSION OF ALUMINUM AND ITS ALLOYS IN SUPERHEATED STEAM

The corrosion behavior of pure aluminum and some of its alloys in superheated steam was found to depend markedly on the method of starting the corrosion test. Pure aluminum samples survived only in tests that were brought to temperature and pressure very rapidly. Resistant Al-- Ni-- Fe alloys performed well only if a relatively slow starting procedure was used, suffering extensive blistering or complete disintegration in a test started rapidly. Over the range of temperature and pressure investigated, 400 to 540 deg C and 150 to 600 psig, with optimum starting conditions both pure aluminum and resistant Al-- Ni-- Fe alloy samples quickly formed a very protective oxide film. Interference colors were noted for exposures of several weeks. Samples surviving a 260-day test at 540 deg C and 600 psig had less than 1-mg/cm/sup 2/ weight gain. Nonresistant alloys disintegrated in short corrosion exposures. A penetrating attack, initiated in only a few spots, rapidly destroyed the samples. The effects of composition, dispersion of second-phsse compounds, hydrogen porosity, and pretreatments were investigated for 5.6% Ni--0.3% Fe-0.1% Ti in 540 deg C, 600-psig steam. It was concluded that porosity produced by corrosion product hydrogen was a major factor in the survival of samples. A mechanism for the rapid penetrating attack was proposed as based on observations made during the study of hydrogen porosity. Pretreatment of resistant alloy samples in dry air at 540 deg C or in high-temperature water at 350 deg C greatly reduced the amount of porosity produced by corrosion in superheated steam. (auth