An electrochemical investigation of erosion corrosion of duplex stainless steel in sea water containing sand particles

Abstract

Passive metals, like stainless steels, are usually protected from corrosion by a thin oxide film on the surface. If the film is stable it will reform spontaneously when it becomes damaged. Erosion-corrosion of passive metals proceeds by the repeated removal and repair of the oxide film. Erosion corrosion behaviour studies were carried out on UNS 31803 grade of duplex stainless steel in seawater using a water jet impingement apparatus. Electrochemical polarisation scans and optical film thickness measurements were utilised to measure the rate of repair of the passive film while varying the fluid flow velocity in seawater with and without the addition of sand particles. The value of the passive current density revealed the severity of the attack, as the fluid velocity was changed on the surface of the material. This demonstrated the pitting of the ferrite phase at 380 mV and breakdown of the austenitic phase at 880 mV. With the presence of sand particles, the passive current density was significantly enhanced and increased up to 7.5 jiA/cm2 for 3 grams and the value stayed constant as the sand concentration increased to 4 grams. Both the current and potential versus time measurements supported the polarisation results. In general, the passive film produced an interface colour of the first or second order. Modelling of the erosion-corrosion behaviour under particle impingement was characterised by developing a relationship between passive film colour thickness, sand particle energy from the water jet impingement system and the electrochemical scans. It was possible to show that the mean charge passed for a single particle impact of 0.01 impact was 6 x.10*12 coulombs and the corresponding absolute current was 6x 10'12 Ampere. Overall, the conclusion drawn from this work was that the UNS31803 duplex stainless steel would exhibit a good resistance to erosion corrosion if used in ship power plants and offshore piping giving a depth of attack between 0.07 and 0.08 mm/year.