Non-destructive evaluation method for far-side corrosion type flaws in oil storage tank bottom floors using the magnetic flux leakage technique

A quantitative procedure based on the magnetic flux leakage technique was proposed to evaluate the size of metal-loss type flaws such as corrosion pits in oil storage tank bottom floors. The approach considers both experimental findings and theoretical modeling of the simplified model flaws of local corrosion or pitting. Experiments were carried out on steel plate specimens, which contained artificial flaws with various depths and widths, such as flat-bottomed cylindrical holes and rectangular grooves in the backside of the specimen plates. The experimental results suggested that the depth of the flaw or residual plate thickness could be estimated by measurements of the strength and distribution of the vertical component of the magnetic leakage flux density, which is affected by far-side flaws. Analytical model calculations confirmed these findings. Moreover, a practical evaluation procedure for sizing flaws or the residual thickness of metal loss area by this technique was developed. The far-side magnetic leakage flux density distributions for conical holes were also measured and calculated using the theoretical model to confirm the procedure, which corrects the amplitude of the calculated far-side magnetic leakage flux density distribution. The far-side magnetic leakage flux density distributions for conical holes could be reproduced using the procedure.