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Inspectioneering Journal

99 Diseases of Pressure Equipment: Phosphoric Acid

By John Reynolds at Intertek. This article appears in the September/October 2004 issue of Inspectioneering Journal

Corrosion from phosphoric acid is another “old knowledge” corrosion issue that effects only a few processes in the chemical and hydrocarbon process industry. In refineries it is largely associated with polymerization units using phosphoric acid as a catalyst. Dry phosphoric acid catalyst is not corrosive to carbon steel but that changes dramatically when free water is present. For nearly all acidic corrosion, water content and temperatures are keys to understanding how aggressive the corrosion is likely to be. Wet phosphoric acid can corrode carbon steel at thousands of mils per year. Where water is present, upgrading to 304L, 316L, or Alloy 20 may be necessary largely depending on temperature. Corrosion can often be found in dead zones and crevices where wet acid can drop out.


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