Development of Proof-of-Concept for Condition-Based Coating Evaluation Sensor, 20-9232Printer Friendly Version
Inclusive Dates: 01/02/01 - 05/02/01
Background - Atmospheric corrosion of steels, aluminum alloys, and Al-clad aluminum alloys is a problem for many civil engineering structures, commercial and military vehicles, and aircraft. Under ideal conditions, a multilayered coating system consisting of a surface pretreatment, a primer coat, and a topcoat is used to reduce corrosion of steel bridge components, automobiles, trucks, and aircraft. At present, maintenance cycles for commercial and military aircraft and ground vehicles, as well as engineered structures, are based on experience and appearance rather than a quantitative determination of coating integrity. Limitations to experience, variations in exposure conditions, and the possibility of extensive corrosion damage occurring in hidden areas such as seams and lap joints are serious restrictions to the present methodology used to determine maintenance schedules and they frequently result in considerable unnecessary costs. Though the ability to determine the condition of an existing coating over time would be beneficial in evaluating maintenance needs and possibly assessing early coating damage or degradation and preventing corrosion of the vehicle or structure, the present suite of sensors designed to detect corrosion can provide only limited and indirect information on the condition of the protective paint coatings.
Approach - The objective was to develop and test an embedded sensor that could monitor the condition of a coating system. To accomplish this objective, a multipin array was mounted to the back side of a series of carbon steel and aluminum alloy panels such that the pins were flush with the front surface of the panels. The panels were then painted with different paint systems including polyurethane and enamel. Additional panels were constructed that were intentionally contaminated with hydraulic fluid prior to painting as well as panels that were subjected to mechanical damage after painting in the form of a scribe or pinhole. The panels were then subjected to an accelerated exposure cycle designed to mimic outdoor environments. Resistance measurements between the sensor pins were used to monitor coating condition.
Accomplishments - In this quick-look project, a new sensor concept (see illustration below) was developed and evaluated to detect and measure the protectiveness of coating systems prior to their complete failure. It was shown that the sensor could distinguish coatings of differing protectiveness and could detect the presence of prior surface contamination and physical defects. Sensor response was also in line with visual inspection of the panels and coating creep back at the physical defects. Unlike other sensor designs mounted to the outside surface or that sometimes cause the corrosion they are trying to detect, this sensor is maintenance free. The current sensor is mounted underneath the coating and has not caused the onset of coating failure nor does it accelerate corrosion by its presence.