Corrosion and Corrosion Fatigue Characteristics of Cast NAB Coated with NAB by HVOF Thermal Spray

Abstract

Cast Ni-Al bronze (NAB) alloy, which is a most common material of ship propeller operating in sea water, is coated with the same NAB alloy by high-velocity oxy-fuel (HVOF) thermal spray process. Application of a HVOF NAB coating to cast NAB can be considered to enhance the resistance to corrosion fatigue failure of a NAB-based ship propeller and repair its damaged location at low cost. Linear sweep voltammetries, potentiostatic corrosion tests, and 80-day immersion corrosion tests are conducted in a 3.5 % NaCl solution to investigate the corroding behavior of HVOF-NAB-coated cast NAB. Corrosion pits and microcracks through which the NaCl solution can easily permeate are developed from many pores included in the HVOF NAB coating. More serious corrosion compared to uncoated cast NAB occurs with producing corrosion products of Cu2O, Al2O3, and Fe2O3. Corro-sion fatigue tests of both HVOF-NAB-coated cast NAB and uncoated cast NAB for reference are also conducted in the NaCl solu-tion. Corrosion fatigue strength of the HVOF-NAB-coated cast NAB is measured as 140 MPa, which is much higher than 90 MPa of the uncoated cast NAB.