Corrosion behavior of prestressing steel in a solution of 20% ammonium thiocyanate

1400-MPa class high-strength PC wires are used for pre-stressed concrete structures. One of trusted hydrogen embrittlement tests for steel is a testing method that is provided in the FIP Report¹ and defined in JSCE S 1201³ for a test procedure. This test includes a constant load test in a 20% NH₄SCN at 50°C for up to 200 hours to measure the rupture time. The test examined corrosion behavior in the early stage of the test when hydrogen concentration in steel would show a rapid increase. It was recognized that dissolved oxygen accelerated a corrosion reaction in pure oxygen atmosphere, but a 6-hour immersion test showed no notable difference in corrosion rate and corrosion potential in air and deaerated atmosphere. Polarization measurements suggested that a cathodic reaction independent of dissolved oxygen exerted a profound effect on a corrosion reaction. The analysis of corrosion products and iron ion led to a presumption that the iron-sulfide and iron oxide would precipitate concurrently in the early stage of immersion in 20% NH₄SCN.