A method of diagnosing pressure boundary integrity of a control includes choosing a control valve having a valve body, determining a velocity profile within the control valve, choosing an area of the velocity profile, choosing a surface on an exterior of the valve body at or adjacent to the area, forming a pad on the exterior surface overlying the point, applying a testing device to the pad, and measuring a reference thickness of a wall of the valve body through the pad using the testing device. The method further includes applying the testing device to the pad at a time subsequent to measurement of the reference thickness, measuring a first thickness, and comparing the first thickness to the reference thickness. The comparison of the first thickness to the reference thickness can be used to calculate the amount of valve body wall lost during using and the remaining life of the control valve.