Corrosion Behavior of Prestressing Steel in a Solution of 20％ Ammonium Thiocyanate

Abstract

High-strength steels or PC steel products are used for pre-stressed concrete structures. Testing of PC steel products in terms of hydrogen embrittlement is specified in the FIP Report, and the test method adopted in ISO 15630 is well known. This test is called the FIP test, whereby a constant load is applied to PC steel in a 20 , 50 ammonium thiocyanate solution. An investigation was carried out on the corrosion behavior of 1,400 MPa class PC steel products in a FIP testing environment. Under this testing environment, a cathodic reaction associated with oxygen consumption and hydrogen generation was identified from the initial stage of dipping, and corrosive products mainly consisting of ferric sulfide were formed on the surface of a specimen detected in the test pieces. From changes in the amount of gas associated with the reaction, it is understood that, together with this corrosion reaction, there also occurred a reaction involving 1 mol of hydrogen generation or 0.5 mol of oxygen consumption for 1 mol dissolution of iron. It is also assumed that the reaction involving hydrogen generation was a cathodic reaction associated with the decomposition of SCN .

Effects of Test Conditions on Corrosion Reactions and Hydrogen Absorption in Hydrogen Embrittlement Tests Using an Ammonium Thiocyanate Solution

“…A time to rupture of approximately 120 h is seen for the FIP test condition of a 20 mass% NH 4 SCN solution. The time to rupture did not change with the 15 mass% NH 4 SCN solution, but with a solution concentration of 10 mass% it increased by 30 h and with the 2 mass% solution it exceeded the test termination time of 200 h. The consumption of NH 4 SCN accompanying corrosion of the steel specimens in the 20 mass% NH 4 SCN solution at 50°C was estimated to be less than 2 mass% even after an elapsed time of 200 h. 6) This indicates that consumption of NH 4 SCN during hydrogen embrittlement testing has a negligible effect on the time to rupture. Figure 2 shows the effect of the NH 4 SCN solution concentration on the hydrogen content and weight loss of the steel specimens due to corrosion during the immersion tests.…”

“…Because all of the test specimens displayed a black color following the immersion tests, it was inferred that iron sulfides and iron oxides formed on the surface. 6) The authors previously reported that the corrosion rate in a pure oxygen atmosphere in a 24-h immersion test in a 20 mass% NH 4 SCN solution was approximately double the rate in a deaeration atmosphere, although the increase in the hydrogen content of the tested steel remained at about 5%. 5) These results indicated that, with a low NH 4 SCN solution concentration where a small quantity of hydrogen is absorbed, dissolved oxygen has a large effect on the corrosion rate of the steel, but the reduction reaction of dissolved oxygen has a small effect on hydrogen absorption.…”

“…aqueous solution. 6) One is a reduction reaction of dissolved oxygen and the other is a cathodic reaction accompanying the decomposition of thiocyanate ion (SCN − ). It has been reported that this cathodic reaction accompanying the decomposition of SCN − promotes hydrogen absorption 5) and simultaneously lowers the reaction rate and forms a corrosion film containing Fe, S and O due to the rise in the solution pH because the reaction consumes hydrogen.…”

“…(3) that accompanies the decomposition of SCN − , the latter two being cathodic reactions. 6) Fe Fe e → + ... (3) Sulfur oxides were also detected in an analysis of the corrosion film, suggesting the possibility that various types of corrosion reactions take place in the NH 4 SCN solution. 6) The cathodic polarization curves reported so far for the NH 4 SCN solution 6,7) have shown that the reactions on the high potential side overlap the anodic reaction of iron; they have not yielded sufficient information about the cathodic reactions, including those in Eqs.…”

“…The structure of the corrosion film that formed had a high concentration of S in its inner layer and a high concentration of O in its outer layer. 4,6) Table 4 shows the change in the pH of the test solution near the counter electrode before and after the polarization measurements in Figs. 4 and 5.…”