A method for measuring a thickness of a deposit layer on a metal or alloy substrate using an X-ray fluorescence (XRF) spectrometer, where the deposit layer includes scale deposits or corrosion products. As part of the method, an elemental composition of the deposit layer or the metal or alloy substrate is measured using the XRF spectrometer. The thickness of the deposit layer is obtained from the elemental composition using a calibration relationship between deposit layer thicknesses and corresponding elemental compositions of the deposit layer or the metal/alloy substrate. The method can be applied to determine the rate of deposit layer formation and evaluate the effectiveness of a treatment.