Paint/Coating Testing & Failure Analysis

Failure Analysis of Paint and Coatings

The stages of analysis performed when conducting paint or coating failure analysis investigation should begin with the collection of background data and sample removal. This step includes site inspection, information regarding the current history of the failure, all relevant record keeping, and records on past failures if applicable.

A preliminary examination of the failed coating and the substrate, as well as a non-destructive examination of the failure, with extensive photographic documentation, precedes any destructive laboratory evaluation and analysis. The preliminary examination does not change or damage the failed coating or substrate in any way.

At this point in the investigation, the specimens should be selected and identified for further laboratory testing and analysis. Management should be notified of any specimens collected from paint and coating failure, as the specimens, including the underlying substrate, are often damaged and of little use after testing.

A macroscopic examination of the surface of the selected specimen begins this stage of analysis, followed by a microscopic examination. A close examination of failed paint and coating chips using a stereo microscope at magnification of 50x or less may reveal that one of the layers is brittle and full of cracks, or perhaps that an entire layer of paint is missing. An examination of failed and non-failed samples may reveal that all of the failed samples are of improper thickness. A microscope at magnifications ranging from 50x to 1000x can be used to examine the cross section of failed paint and coating samples for voids or inclusion, as well as observation of underlying corrosion products on substrates.

A chemical analysis of the paint or coating, as well as the substrate and corrosion products is usually the next step. Chemical analysis techniques typically used in the laboratory for paint and coating failure analysis are Fourier transform infrared spectroscopy (FTIR) for organic functional group analysis, scanning electron microscopy (SEM) with associated energy dispersive x-ray spectroscopy (EDS) for elemental analysis, and x-ray photoelectron spectroscopy (XPS) for surface elemental analysis.

Accelerated environmental exposure tests, such as salt spray (fog) tests, humidity tests, and ultraviolet light (QUV) exposure tests can help to confirm the proposed failure mechanism of a painted or coated substrate sample. Accelerated exposure testing can be complemented with electrochemical impedance spectroscopy (EIS). In the EIS technique, capacitance and electrical properties of the coating are measured as a function of time. If the impedance ratio does not change as a function of time, then one can, with high degree of confidence, conclude that the coating is not altered and performs very well under actual service conditions.

Analysis of the evidence; and a review of the existing data and documentation are the final stages of failure investigation. All information is gathered and analyzed to form a determination on the mode and probable cause of the failure. Identification of the mode and cause of failure provide the source for recommendations for corrective action. A final report including all relevant data, analyses, and recommendations are compiled and presented to the client. In litigation investigations, the client may not be interested in the recommendations section of the report.