Paint and Protective Coating Failure Analysis

To investigate a failure, and analyze the conditions that promoted the failure, important information must be collected on the failed paint or coating. Background information on the coating type and application procedure, the service history and environment, and physical evidence of the failed coating are necessary to determine why, how, when, and where a failure may have occurred. If these answers are provided during the course of the investigation, future failures may be better understood or possibly prevented.

The conditions that promoted the failure are essential in identifying the underlying factors that may have initiated the failure. Other elements that may not be readily acknowledged in failure analysis, yet are no less important, are common sense, a critical and unbiased mode of thinking, experience, knowledge, and experimental observation.

Provided in this survey is a step by step approach to paint and coating failure analysis investigation. The accepted theories and mechanisms, which cause paints and coatings to fail, will be explored in this paper.

FAILURE ANALYSIS

A failure analysis investigation is much like the work of a detective. Clues or relevant facts pertaining to the investigation must be gathered, analyzed, explored, and studied to make a knowledgeable determination. As in the case of a good detective, first hand field experience is of the utmost important, yet academic studies are also essential.

Failure Analysis – Sequence of Events

Justification for conducting paint or coating failure analysis investigations is the most important issue for a failure analyst. Corrosion protection, aesthetic, production, or litigation related purposes provide excellent examples for justification.

If the investigation is fully justified, the method for evaluation proceeds with the second step in the process. This step involves gathering relevant information and facts concerning the failure. The questions listed serve as a guide to follow during the investigation. When the information has been obtained it must be carefully organized, labeled, and documented in a logical format for future reference.

The failure analyst should question why failure occurred, how to get the building, facility, or equipment repair coated quickly if necessary, how to prevent a recurrence of the problem, and if more information is needed, how can the information be readily obtained.

With these steps taken a plan of attack can be formed. This is the single most important step in the method of evaluation. A logical plan for the investigation to follow must be developed and implemented. Each investigation will be different from the last and many variables will make it necessary to make decisions based on the investigation at hand. If an analyst is hasty in his decisions and does not have a solid plan the entire project may be ruined. By simply cutting or analyzing a sample carelessly an analyst could destroy his only useful evidence.

The stages of analysis performed when conducting a 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 and statistics of failure 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 a paint and coating failure, including the underlying substrate, are often damaged, and of little use after testing.

There is a wide variety of testing methods currently available for failure analysis of paints and coatings. Sophisticated and highly calibrated laboratory equipment can detect the slightest imperfections on a specimen, and accurately identify the inherent characteristics.

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 magnification 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.

Physical testing provides important characteristics of a paint or coating specimen which may reveal primary causes for the failure. Important physical tests include thickness testing, pin hole testing, adhesion testing, determination of the plane of delamination, hardness testing, and surface roughness (profile) testing. Pin holes are caused by poor application technique, solvent evolution from the film, and corrosion due to trapped materials. Poor adhesion is caused by improper surface preparation procedure, as well as incompatibility of coating layers or of the primer with the substrate. The determination of the plane of delamination of a failed coating is critical to ascertaining the possibility of coating layer incompatibility or improper surface preparation.

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, gas chromatography – mass spectrometry (GM-MS) for organic compound separation, identification and quantification, differential scanning calorimetry (DSC) for melt range and thermal properties, scanning electron microscopy (SEM) with associated energy dispersive x-ray spectroscopy (EDS) for elemental analysis, and Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) for surface elemental analysis.

Fourier transform infrared (FTIR) spectrometers record the interaction of infrared radiation (light) with experimental samples, measuring the frequencies at which the sample absorbs the radiation and the intensities of the absorptions. Determining these frequencies allows identification of the sample’s chemical makeup, since chemical functional groups are known to absorb infrared radiation at specific frequencies. FTIR is most often used to identify paint binders and differentiate between general classes, such as oil based polyesters or acrylic latexes. Some pigments and inorganic materials can also be identified in a general sense through this method.

In the simplest terms the gas chromatography – mass spectrometry (GC-MS) instrument represents a device that separates chemical mixtures, primarily organic mixtures, (the GC component), a device for ionizing the separated chemical compounds from the mixtures, and a very sensitive detector (the MS component) with a data collector (the computer component). After the ions are separated according to their masses, they enter a detector and then on to an amplifier to boost the signal. GC-MS can be useful in paint failure analysis to determine presence of certain additives and low-molecular weight materials not easily detectable by other methods.

Differential scanning calorimetry (DSC) is used to determine the melt range and other thermal properties of polymers, which comprise the majority of binder for organic coatings and paints. Melt temperature and ranges can be influenced by properties such as molecular weight, crystallinity, and chain branching, which in turn can affect physical performance of the polymer in use. In paint, DSC can be used to compare degree of cure, usually of two component systems such as epoxies or polyurethanes. It can provide a more quantitative measure than other tests such as solvent rub.

The scanning electron microscope (SEM) is a microscope that uses electrons rather than light to form an image. There are many advantages to using the SEM as an adjunct to the optical (light) microscope. The SEM has a large depth of field, which allows a large amount of the sample to be in focus at one time. The SEM also produces images of high resolution, which means that closely spaced features can be examined at a high magnification. Preparation of the samples is relatively easy since most SEMs only require the sample to be electrically conductive. SEM is valuable for looking at small features that would indicate proper surface preparation, such as swirl lines from sanding or coarse texture from an acid etch, especially in smooth surfaces where such preparation is necessarily low profile. SEM can also be used to look for particulate contaminants on contact surfaces of paint chips.

Energy dispersive x-ray spectroscopy (EDS) systems are used in the characterization of materials through the use of ionizing radiation to excite a sample. This excitation generates x-ray energies that identify the elemental composition of the sample. Using x-ray detection equipment to count the number of x-ray photons emitted by this technique, an EDS system is able to characterize and quantify in an approximate manner the elemental composition of the sample. Either used in conjunction with SEM imaging or on its own, the most common use for EDS in paint analysis is a general determination of inorganic materials to confirm pigment content. EDS can also be useful for looking for contaminants and determining their general composition, and to differentiate among certain elements that appear in similar areas in FTIR spectra.

Auger electron spectroscopy (AES) determines the elemental composition of conductive and semi-conductive surfaces, and can provide elemental depth profiles through sputtering. This information can then be utilized to solve problems associated with coating surface appearance, cleanliness and bonding of conductive coatings. Additionally, corrosion products may be identified. In principle, an electron beam bombarding a solid surface excites electrons from core electronic energy levels of atoms. The kinetic energy spectrum is used to identify the atom of origin and its concentration. X-ray photoelectron spectroscopy (XPS) provides similar analyses offered by AES and uses x-ray to excite core electrons, but XPS can be performed on a non-conducting surface, providing a definite advantage for organic coating materials and paints. XPS also may provide more detailed information on the binding state of an element, which provides the chemical bonds and the identity of compounds, which EDS and AES cannot provide.

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). The organic coating resistance generally degrades with time. The degradation is associated with corrosive ions and water penetration into the coating, transport of ions through the coating, and subsequent corrosion reactions at the polymer–metal interface. Typical standard coating immersion tests can take hundreds or thousands of hours. However the EIS technique can provide reliable data on performance in rather short time.

In the EIS technique the capacitance and electrical properties of the coating are measured as a function of time. Electrochemical impedance is measured by applying an AC potential to an electrochemical cell and measuring the current. The response to this potential is an AC current signal, containing the excitation frequency and its harmonics. The advantage of this technique is that acceleration of corrosion during the test will not occur such that conditions will result in the failure of coating system. The test will provide reliable data in a short time of exposure. The data from this test can also provide data for calculating permeation rate and minimum coating thickness and cure schedule requirements for adequate corrosion resistance. In one case physical testing and chemical testing of the paint system indicated no reason for failure and corrosion of the component in service. However, EIS testing exhibited %20 drop in percent ideal (a measure of degree of protection) in five days of testing. The EIS results provided an explanation for the failure. Although there was no physical or chemical defect in the painted metal, the paint layer did not act as a barrier in that particular service exposure.

The careful application of EIS technique in conjunction with other methods of testing can be tremendous aid in selection of coatings for given application. For example, salt spray testing in accordance to ASTM B 117 is frequently used to measure the corrosion resistance properties of the coating in the presence of corrosive conditions containing chlorides. Although salt spray testing accelerates the corrosion attack experienced on coated panels, it can sometimes take over 2000-5000 hours until visible signs of degradation appear. A perfect coating will produce a straight line when evaluated with this method, no change or little change in impedance of a good coating occurs as a function of time, so as the coating degrades the graphs begin to slope. By calculating percent ideal, we are able to determine which test panels are performing the best. If the impedance ratio does not change with time, then one can with high degree of confidence conclude that the coating is not altered and performs well under actual service conditions.

A paint failure should be looked at from both the paint physical and chemical characteristics as well as the substrate corrosion perspectives. This will help determine which technique will provide the most useful information, given the nature of the failure. For example, gas chromatography is excellent at detecting solvents, but would be of little use in determining why an alkyd resin failed on a galvanized substrate.

Another example is that only EIS and exposure tests can distinguish between storage-related problems and paint application/ formulation problems in white rust formation of painted galvanized steels. Traditional approaches cannot make these distinctions. Electrochemical evaluation of paints and coatings provide a very powerful investigative technique for root cause determinations. The degradation of an organic coating on a metal substrate will exhibit various stages of coating failure. These insights could save unnecessary effort and expense by selecting the proper methods of testing and analysis.

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.

Collection of Background Data

The failure analyst should determine when, where, and how the paint or coating failure occurred. Interview all users and operators involved in the failure with point-related questions. Examples of point-related questions include “how was the coating-substrate handled after failure?” and “was it protected?”

Sample Removal

The decision to remove a sample specimen of a paint or coating, or of the underlying substrate, is a very important part of the failure analysis investigation. Sample selected should be characteristic of the coating and/or substrate and contain a representation of the failure or corrosion attack. For comparative purposes, a sample of the intact coating and/or substrate should be taken from a sound and normal section. In conjunction, for a complete microscopic and chemical evaluation and analysis, samples from the failure, adjacent to the failure, and away from the failure are necessary.

The sample must be removed without changing the surface conditions or characteristics of the sample, nor inflicting physical damage of any kind. The sample is the basis on which the investigation and analysis rely, and extreme care must taken not destroy any of the sample’s properties.

Upon removal of the sample, the exact location where it was obtained must be documented both in writing and by photography. Any corrosion product found on the coating or the substrate should also be collected and examined. If necessary, the corrosion product may be removed carefully from the substrate and sent with the removed sample. When the sample has been removed from the structure or the component, it should be carefully packaged in a water-tight container appropriate to the size on the sample, identified and labeled.

PAINT AND COATING FAILURES

The majority of paint and coating-related failures can be attributed to six primary causes. These causes are as follows.

• Improper surface preparation – the substrate surface is not adequately prepared for the coating that is to be applied. This may include cleaning, chemical pretreatment or surface roughening.
• Improper coating selection – either the paint or coating selected is not suitable for the intended service environment, or it is not compatible with the substrate surface.
• Improper application – this can be a problem with either shop-applied or field applied coatings, and occurs when the required specifications or parameters for the application are not met.
• Improper drying, curing and over coating times – again, this problem relates to a lack of conformance to the required specifications or parameters.
• Lack of protection against water and aqueous systems – this is a particularly serious problem with aqueous systems containing corrosive compounds such as chlorides.
• Mechanical damage – which results from improper handling of the painted or coated substrate, resulting in a breach in the paint or coating.

There are innumerable possible failure modes which can result from these primary causes. For the purposes of this review paper, the failure modes will be divided into three general categories, as follows.

• Formulation-related failures.
• Substrate-related failures.
• Physical defect-related failures, including blistering.

These three general categories of failure modes will be described briefly.

Formulation-Related Failures

There are many types of paint and coating failures for which the coatings or corrosion engineer has little or no control over. These types of failures are related to the formulation of the coating itself. If the coating system that is selected by the engineer is formulated inadequately, the coating will most likely fail regardless of all efforts made in an optimal application. These formulation-related failures occur as a result of the ingredients used and their formulation in the paint or coating. These ingredients include the resins used, the pigments used, as well as the solvent formulation. The specific types of formulation-related failures include chalking; erosion; checking; alligatoring; cracking; mud crack; wrinkling; biological failure; and discoloration for organic coatings; and checking; mud cracking; and pinpoint rusting for inorganic (zinc) coatings.

Substrate-Related Failures

A substantial percentage of paint and coating failures are related to the substrate to be coated and its proper preparation prior to coating. To eliminate this class of paint and coating failure, it is imperative that the painters and coating applicators take great care in following specified methods of surface preparation. There is no substitute for proper surface preparation if long service lifetime is expected from the paint or coating. The specific types of substrate-related failures include previously used steel; galvanized or metallic zinc surface; aluminum; copper; wood; and concrete.

Physical Defect-Related Failures

Many specific types of physical defects have been categorized and studied by the paint and coatings industry and by coatings and corrosion engineers. Many of these physical defect-related failure types overlap with the formulation-related failure and substrate-related failure types discussed above. However, these physical defect types and their nomenclature as discussed here are traditionally considered by the paint and coatings industry, and they merit a separate category. The specific types of physical defect-related failures include blisters; bubbles and craters; color mismatch; dirt; fisheyes; gloss variations; mottle; orange peel; runs, sags, and curtains; paint adhesion loss; soft paint films; solvent popping, boiling, and pinholes; and solvent wash.

Blistering

Blisters are local defects that form because of the pressure exerted by an accumulation of water or aqueous solution at the coating-substrate interface in conjunction with loss of adhesion and distention of the coating. At these local regions, corrosion of the substrate may occur. But the loss of coating adhesion is actually connected with the development of a cathodic area under the coating adjacent to the defect. Oxygen also permeates the coating while ionic materials are leached from the substrate or from the coating and these all concentrate to make an electrochemical corrosion cell beneath the blister. Therefore blisters are an early sign of corrosion but are often neglected. Conversely, the elimination, reduction, or delay in blister formation will delay the onset of corrosion of the steel substrate. However, even with correctly applied cathodic protection, shielding (due to coating disbondment) can render CP locally ineffective and corrosion can still occur.

There is no consensus on the number of different forms and the actual mechanisms for blister formation but the most likely possibilities are identified below. In general, the mechanism of blistering is attributed to osmotic attack or the presence of defects in the coating interfacial region, in combination with the influence of moisture. The following sequence of events leading to blistering is general to most types:

1. The film absorbs water from solution, possibly containing dissolved salts.

2. Once sufficient chloride ions pass through to the underlying metal, primary corrosion is initiated at sites along the interface, particularly at existing defective areas or areas of substrate contamination.

3. As corrosion proceeds at the anodic sites under the film, hydroxyl ions build up at cathodic sites.

4. The alkaline environment at the cathodic sites weakens or destroys the adhesion of the film while producing osmotically active substances at the coating/metal interface.

5. The presence of these active substances at the interface causes osmotic (or endosmotic) passage of water from the coating surface to the interface resulting in pressures that exceed the interfacial strength of the film and eventually the fracture strength of the film causing further delamination or coating rupture.

Four mechanisms are generally proposed to explain blister formation: volume expansion due to swelling, gas inclusion or gas formation, electroendosmotic blistering, and osmotic blistering. Some sources consider phase separation during film formation resulting in the presence of a hydrophilic solvent a separate mechanism, however it is really a subset of osmotic processes. Furthermore, this mechanism is not applicable to FBE coatings since solvents are not involved in the application of the epoxy resin. Interestingly, the first two mechanisms result in loss of adhesion of the coating (at least locally) prior to the onset corrosion, while the later processes including cathodic delamination are the result of corrosive action at the interface.

A recent major client produces tank cars for transportation of bulk chemicals. They have a huge facility in which they prepare the cars for periodic interior painting and then actually do the complete complex procedure of painting them. With several thousand cars on the rails, this process represents a major investment of time and capital. But the top coat on many of the cars was peeling. MATCO was hired to determine what the root cause of this failure was and to prescribe an improved procedure to avoid the problem in the future.

After visiting the site and photographing the entire process, MATCO scientists were provided with circular “plugs” from the walls of dozens of the cars, representing a broad spectrum of quality. Many of these “plugs” were prepared as metallurgical cross-sections, to determine the nature of the bond between metal, primer and top coat. These sections were analyzed by light and scanning electron microscopy. MATCO made a wide range of trial panels under varying conditions to duplicate the failure conditions.

It developed that microscopic observation of the failed debonded primer surface under grazing-incidence illumination revealed that the primer always showed a peculiar circular structure, indicating that the paint had failed to flow out properly. The micrograph at the top of this page shows those circular separated phases at the surface of the failed primer. This was caused either by an initial improper rate of thinning or by application conditions that caused too much thinner to evaporate before the paint hit the metal surface. This degraded the bond between primer and top coat. The problem was too-lax controls on application conditions, including distance from spray gun to surface, ambient temperature and mixing of thinner and bulk paint.

One of MATCO's strengths is the use of cross-section microscopy to detect interface problems in paint and coating systems. Metallurgical and scanning microscopy are both used routinely, in contrast to the approaches of many other prominent laboratories.

This is typical of the breadth and depth of MATCO’s investigations of problems in paints and coatings. Whatever your concerns, MATCO can help you put an effective procedure in place to get back on track.

Our team includes NACE Certified Corrosion / Cathodic Protection/ Materials Selection/ Design / Coating Specialists (*)and other materials experts includes:

Dr. Mehrooz Zamanzadeh(*)             Mr. Geoff Rhodes (*)

Dr. Don Gibbon                                Mr. Kevin Groll (*)

Dr. George T. Bayer                         Mr. Ed Larkin

Ms. Amy Drabant

Dr. Huiping Xu

Please review our newest internal publication, "Failure Analysis of Coatings" by Dr. George T. Bayer and Dr. Mehrooz Zamanzadeh

Painted Galvinized Steel Case HIstories, by Dr. Mehrooz Zamanzadeh and Dr. George Bayer presented at PACE 2006.