Bentonite films to protect steel from oxidation

Bentonite films protect heated steel from oxidation when channels through them are absent or are too narrow for oxygen to pass. Ca, Mg and Fe‐bentonites are flocculated and thus produce films containing channels. Na bentonites, being well‐dispersed, give relatively impervious films. Films can be effectively scrubbed from the steel if their montmorillonite expands with water, despite prior heating, but the scrubbing of resistant films with cleansing solutions suggests that wettability is also significant. Na‐Ca‐bentonites showed good overall behaviour when the proportion of Ca was low. A small critical proportion of Ca accelerated thixotropic setting and thus was likely to give a less permeable film. Progressive substitution of Na by Ca produced flocculation, permeable films and difficulty in scrubbing the clay from the steel. This difficulty supervened at different Na/Ca ratios for different bentonites. Heating was to 700° momentarily. Lower peak temperatures should make a wider range of bentonites acceptable.