THE EFFECT OF HYDROQUINONE AND METHOXYPROPYLAMINE ON THE PITTING CORROSION BEHAVIOR OF A-470 LOW-ALLOY TURBINE DISC STEEL

Electrochemical techniques have been used to study the pitting corrosi on behaviour of A-470 turbine rotor disc low alloy steel (LAS) in a si mulated aggressive turbine environment containing 2 ppm NaCl + 2 ppm N a2SO4 + 2 ppm NaOH + 5 PPM SiO2 (30-degrees-C) in the presence of hydr oquinone (HQ) and methoxypropylamine (MPA), alone or in combination. A ddition of HQ to N2 purged solutions shifts the corrosion potential in the positive direction and spontaneous passivation is observed at con centrations of approximately 100 ppb. HQ concentrations as low as 150 ppb can induce pitting and Scanning Electron Microscopy studies reveal ed these pits to have been formed primarily at MnS inclusions. Potenti odynamic polarization has been used to determine the pitting potential (E(pit)) of the alloy in N2 purged solutions containing different con centrations of MPA between 1 x 10(-4) and 1 x 10(-3) M. E(pit) increas ed with increasing MPA concentration and immunity to pitting corrosion is evident at concentrations above 4 x 10(-4) M. Surface analysis stu dies show this inhibition to be due to the chemisorption of MPA on the metal. In case of HQ/MPA mixtures, the pitting corrosion of the alloy by 200 ppb HQ is completely inhibited by 2 x 10(-4) M MPA. The normal practice of adding amines to adjust the pH of turbine feed water acts to counteract the pitting characteristics of HO at higher concentrati ons.