Biogenic thiosulfate and oxalate in paper machine deposits connected to corrosion of stainless steels

Pick-up felts and deposits in wet end splash areas and in storage tanks of paper machines contained large amounts of thiosulfate (80–16 000 mgkg−1, wet weight) and oxalate (20–2300 mgkg−1, wet weight). These anions were absent (<5 mgl−1) in process water and found in raw materials only in low concentrations (5–10 mgkg−1, wet weight). Similar and larger amounts of thiosulfate, sulfite and oxalate were generated de novo from sulfate and sheets of chemical pulp in a splash area simulator fed with artificial paper machine white water in the laboratory. Stainless steel UNS S30400 coupons placed in the simulator developed significant corrosion pits within 4 weeks. Pitting was most extensive under or vicinal to patches where large amount of thiosulfate and oxalate accumulated in the pulp sheet. Pitting of UNS S31600 occurred only vicinal to the pulp sheet. The pitted steel surfaces accumulated large amounts of silicon containing material. The biogenic conversion of sulfate into compounds of lower oxidation states occurred more extensively on stainless steel UNS S30400 than on UNS S31600.