INDUSTRIAL PROTECTIVE COATING TRADEOFFS: UNDERSTANDING WHY INDUSTRIAL COATINGS CAN BE COMPLICATED

Transcription

1 INDUSTRIAL PROTECTIVE COATING TRADEOFFS: UNDERSTANDING WHY INDUSTRIAL COATINGS CAN BE COMPLICATED J. Peter Ault, P.E. Elzly Technology Corporation Abstract: Industrial protective coatings can be surprisingly complicated for the inexperienced user or specifier. For example, when fabricating complex structures the simple question of when during fabrication to paint can have an array of interesting cost and performance implications. Other issues include tradeoffs associated with degree of surface preparation and inspection for coating coverage. The paper will explore some of the reasons why they can be complicated in the context of four issues that can impact an industrial protective coatings project. Introduction When specifying or executing a protective coating project, it is important to think about why a particular requirement is being invoked. The tendency to reuse specifications from one project to the next can often ingrain expensive, non-value adding activities and requirements into a protective coatings project. Continuous improvement processes show the value of re-visiting standard specification items and revising them based on the realities of the executed project. When reviewing a specification, it is useful to consider the following questions for each requirement, no matter how insignificant it may seem: Is this requirement for something which is outside of your control to change (e.g., compliance with a law or corporate rule)? Is there data showing a favorable cost-benefit relationship for the requirement? If so, how applicable is it to the present job? Is this requirement simply a preferred alternative that has become common practice in the industry? Is it conceivable that an alternative approach could be developed which presents a better value proposition? Have similar projects resulted in a waiver of this requirement? If so, could the waiver be integrated into the new specification with minimal performance risk? Could the requirement be modified to give potential bidders more flexibility in executing the work without impacting the project value? The following sections discuss four technical requirements with the purpose of helping the reader appreciate the nuances of the requirement and how they may impact a project. There is not necessarily a right or wrong approach to any of the technical requirements. Each requirement is a point on a continuum of cost-benefit tradeoffs.

2 Visible Contaminants (SSPC SP-Grade) Surface preparation is commonly specified using a series of visual standards published by SSPC which describe various levels of cleaning. 1 Key differences among these standards include the amount of residual visible contamination that remains on the substrate following preparation. It has become generally accepted in the industrial coatings industry that coatings will perform better when applied to a cleaner surface. Figure 1 shows the probability of failure for two different surface preparations, SSPC SP-2 versus SP Failure was defined as the point in time where the coating exhibited an ASTM D 610 rating of 7 or less. A range of coatings were exposed in different environments and the number of panels that had exceeded the failure criteria were recorded as a function of time. The data presented is after nine years of exposure. The test protocol called for pre-rusted surfaces prior to hand tool cleaning; the remaining surfaces were covered with mill scale prior to abrasive blasting. The data reflects the decrease in coating performance associated with coating a pre-rusted, tool cleaned surface. This would be relevant to a situation where a small section of an otherwise well prepared surface is touched up using hand-tool preparation. Percent Failed 100.0% 90.0% 80.0% 70.0% 60.0% 50.0% 40.0% 30.0% 20.0% 10.0% 0.0% Years Exposure SP-10 SP-2 Figure 1. Effect of SSPC Surface Preparation Grade - Atmospheric Exposure The SSPC SP-2 (Hand Tool) prepared surfaces failed much quicker than the remaining surfaces. The failure rate of the coatings applied over the SP-2 substrate are about 10 times higher for the first 2 years of exposure and are about 3 times higher over the longer exposure. This magnitude of service life reduction 1 Systems and Specifications: SSPC Painting Manual, Volume 2, 2005 Edition 2 Performance of Alternate Coatings in the Environment (PACE), Federal Highway Administration Report FHWA- RD , June 1989.

3 is corroborated by work done for the Federal Highway Administration. 3 Work performed for the United States Navy also has demonstrated a significantly shorter life for coatings applied over SP-2 versus SP-10 surfaces. 4 SSPC surface preparation standards SP-10, SP-6 and SP-7 (Near-white Metal Blast, Commercial Blast and Brush-off Blast, respectively) are progressively lower levels of abrasive blast cleaning. SP-10, near white metal blast cleaning requires 95% of the surface be free of all visible residue and only staining permitted on the remainder. SP-6, commercial blast cleaning requires two thirds of the surface be free of all visible residue and only staining permitted on the remainder. SP-7, brush off blast cleaning requires that all except tightly adhering residues be removed while uniformly roughening the surface. The difference in performance over these varying degrees of surface preparation has been the subject of relatively few studies. One study 5 showed that performance was equal or better over the lesser surface preparations on average. Figure 2 shows the data from the report. G40 Grit, All Coating Systems Months to Failure SP-7, Brush Off Blast SP-6, Commercial Blast 20% Failure 50% Failure SP-10, Near-White Metal Blast Figure 2. Data showing performance as a function of surface preparation. The New Jersey DOT has an ongoing evaluation of various bridge coatings on the Thomas Mathis Bridge which carries State Route 37 over Barnegat Bay in central New Jersey. 6 This project involved application of a different coating system to each span of the bridge. Each span was entirely painted with the test coating system (bearings, stringers, diaphragms, etc.). The author has participated in several inspections of these structures. Figure 3 shows performance plots for two spans with different degrees of surface preparation. The data show that coating performance over a commercial blast was actually better than the coating performance over a near-white metal blast. 3 Environmentally Acceptable Materials for the Corrosion Protection of Steel Bridges, Federal Highway Administration Report FHWA RD , January Performance of Selected Coatings over Hand-tool Cleaned Surfaces, Ocean City Research Corporation Final Report to Naval Sea Systems Command, July Performance of Alternate Coatings in the Environment Part I: Ten-Year Field Study; SSPC: The Society for Protective Coatings, Pittsburgh, PA, A. Chmiel, V. Mottloa, and J. Kauffman, Structural Coating Evaluation in New Jersey, Research News, Journal of Protective Coatings and Linings, January 1989, pp

4 Organic Zinc/Epoxy/Urethane Coating System Performance on Mathis Bridge PErformance Rating (10=Perfect) SP-10, Near-white Metal Blast SP-6, Commercial Blast Years in Service Figure 3. Performance of OZ/E/U coating systems over commercial and near-white metal blast. The data presented above confirm the industry consensus that abrasive blasting will lead to better corrosion protection than hand tool cleaning prior to coating application. However, the data challenge the notion that higher levels of abrasive blast cleaning result in improve corrosion protection. Obviously, each of these degrees of surface preparation has a different cost. That cost includes not only the direct cost of doing the work but also the soft costs associated with downtime of the asset being coated. The coating project designer should consider the cost-benefit of employing alternative surface preparation strategies. An FHWA report discusses the life cycle cost of various approached to protective coatings. 7 Surface Profile Surface profile is a parameter which is commonly and easily measured as part of the surface preparation process. The surface profile is generally believed to promote adhesion and thus good coating performance. Too large of a profile (typically over 4 or 5 mils) can be a concern since peaks may not be completely covered by the coating. Insufficient profiles (typically under 1 or 2 mils) are thought to be inadequate to promote adhesion. Most surface profile measurements are made using a replica-tape type process and are basically intended to illustrate the mean peak-to-valley height of the surface profile. Surface profilometers can provide substantially more information about surface roughness than simple peak to trough distance that is typically measured in the coatings industry using replica tape or comparators. These profilometers are not new, as far back as 1974 they were being investigated for determination of surface profile. 8 Older technology was not suitable for field use in the protective coating industry. As the instruments have become more durable, automated, and less expensive, their use is being revisited. ASTM D , Standard Test Method for Measurement of Surface Roughness of Abrasive Blast Cleaned Metal Surfaces 7 Guidelines for Repair and Maintenance of Bridge Coatings: Overcoating, C.L. Farschon, R.A. Kogler, and J.P. Ault, August 1997, FHWA-RD , 95 p 8 Keane, et. al., Surface Profile for Anti-Corrosive Paints, SSPC publication 74-1.

5 Using a Portable Stylus Instrument was recently published. Surface profilometer data has the potential to discriminate among various shapes of profile (e.g., rounded versus angular) as well as profile depth. Industry practice commonly recommends a surface profile in the range of 1 to 4 mils from peak to valley. It is generally believed that the risk of too smooth a profile is greater than the risk associated with too high of a profile. Higher profiles can increase cost (increased time to clean and increased coating usage) without substantially improving performance. Furthermore, there is the risk of inadequate coating coverage over a high profile. An SSPC publication comments, Within the normal range of abrasives used in good blast cleaning practice, the usual variations in resulting profile had a relatively minor effect on the performance of typical generic coatings exposed in typical environments. 9 This document was concerned with the performance of vinyl and alkyd coatings over surface profiles in the 1.5 to 3 mil range. Concerning profile an industry discussion concluded 10 that (1) often profiles in the 2 mil range are an adequate target range, (2) that the profile may need to exceed 2.5 mils for high build epoxy coatings, and (3) that the shape of the profile is more important than the depth of the profile. There is data which suggest that deeper blast profiles help constrain the coating from swelling, thus limiting the quantity of liquid which will be taken up by the coating. In his work on cargo coatings, Mills discusses the relationship between absorption of electrolyte by a coating in a constrained geometry (i.e., over deep blast profile) versus an unconstrained (smooth surface) geometry. 11 A coating which is less constrained will absorb more of the electrolyte than a constrained coating. In Nguyen s unified model for organic coating degradation 12 he discusses first water uptake, followed by ionic migration. Conceptually, a coating which can take up more water would likely provide more ionic pathways. What is traditionally assumed to be an adhesion problem over lower surface profiles may also have an element of failure due to increased permeability of the coating. In his work with profilometer measurement techniques, Roper demonstrated that peak density is a parameter that correlates better with adhesion values than depth of profile. 13 In his work, test panels were prepared with the same nominal peak-to-valley profile of 2.5 mils but with peak count (defined as the number of peaks per linear inch along the face of the panel) of 56 peaks per inch for the low peak count and 126 for the high peak count. Figure 4 summarizes the improvements in three key performance parameters observed by Roper. The improvement in performance is significant (i.e., a factor of 1.5 to 2) and consistent for polyurethane and epoxy type coatings evaluated in cyclic test and immersion environments. 9 Surface Profile for Anti-Corrosion Paints, Steel Structure Painting Council, Problem Solving Forum, Protective Coatings Europe, December Mills, George, The Testing of Anti-Corrosion Tank Linings for Marine Tankers, paper presented at Corrosion/ T. Nguyen, J. Hubbard, & J. Pommersheim, Unified Model for the Degradation of Organic coatings on Steel in a Neutral Environment, Journal of Coatings Technology, Volume 68, Number 855, pp Roper, H. et. al., The Effect of Peak Count of Surface Roughness on Coating Performance, JPCL, June 2005.

6 Performance Improvement for Coatings Applied over High Peak Count Surface 200% Improvement versus Low Peak Count 150% 100% 50% 0% Pull off strength after Exposure Pull off failure at substrate Maximum scribe undercutting Performance Parameter Figure 4. Influence of Peak Density on Coating Performance Figure 5 shows failure distributions for SP-5 and SP-10 prepared surfaces using either abrasive shot or grit from the SSPC PACE study. 14 The study provided data on the performance of different coatings applied over different SSPC grades of surface preparation exposed in different atmospheres. In the study, test panels were prepared to either the SP-5 or SP-10 surface preparation using shot or angular grit. The data suggest that the coatings applied over the shot surfaces will have less service life than those applied over the grit blasted surface. This difference is as much as 100% in the early years and about 30% at 9 years. Alternatively, the service life is reduced by approximately 2 years when shot is used rather than grit (i.e., the distance between the red and blue lines at any given failure percentage). A surface prepared with shot probably has a lower peak density than a grit blasted surface. Thus, this observation corroborates the observations by Roper i.e., peak density is an important surface characterization measure. The effect of surface profile shape is critical in applications where shot is used in place of or in conjunction with grit abrasive. Shot abrasives are typically used in recycled blast equipment. They feed into the equipment better and cause less wear on the impellers. Unfortunately, the rounded profile which results from shot blasting can result in lower coating adhesion. Grit abrasive is generally mixed with shot to balance the need for an angular profile with operating efficiency of the machine. As the grit breaks down, the operating mix of abrasive changes. One benefit of monitoring peak density is to help determine when the operating mix does not contain sufficient grit. 14 Performance of Alternate Coatings in the Environment (PACE), Federal Highway Report FHWA-RD , June 1989.

7 Shot v. Grit - Atmospheric % Failed 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% SP-5/SP-10 Shot SP-5/SP-10 Grit 0% Years Figure 5. Effect of Shot v. Grit on Coating Life With respect to profile, a study by SSPC 15 identified that critical film thicknesses of 1 to 2 mils measured over the peaks was necessary to eliminate the early rusting failures of vinyl and alkyd coatings associated with insufficient coverage of the surface. The data suggested that the lower value of 1 mil was adequate when the surface profile was approximately 2.5 mils but a thickness over the peaks of close to 2 mils was determined necessary when the surface profile was approximately 5 mils. This relates to the propensity for holiday formation where the coating is thin over the peaks of the profile (the effect of holidays is discussed in the next section). Over-blasting (e.g., too deep of a profile) should be controlled as it will increase the cost of the abrasive used and will also increase the volume of primer needed to provide a pore free prime coat. The discussion touches on a range of tradeoffs associated with surface profile. The coating specifier must understand the cost impact and potential benefits of specifying a particular blast profile. For some projects, the specifier can be fairly certain that the contractor will request a waiver for high surface profile. By broadening the range of acceptable profile and/or changing the requirement form a profile depth to another metric (e.g., peak density or coverage of peaks) which may be more cost-effective. Coating Holidays The electronic detection of bare areas and holidays may be one of the most effective but under-utilized QA processes. While it is common industry practice to conduct a visual survey of a coated area and identify missed areas for touchup, electrical holiday inspection is often not conducted because of cost 15 Surface Profile for Anti-Corrosion Paints, Steel Structure Painting Council, 1974.

8 (inspection cost and repair cost) and concern that it is an unreasonable requirement. This practice implies that uncoated areas are not important if they are sufficiently small, yet coating delamination and rusting begins at small areas of exposed steel. Once a coating defect occurs which exposes the substrate, corrosion undercutting from the defect becomes a factor impacting coating service life. Undercutting rates from intentional scribes is a metric often used in panel tests. Recent use of fluorescent dyes in primers is an attempt to improve the inspectors ability to detect small uncoated areas (both primer and topcoat) during a visual survey. It is a risk-reward tradeoff to justify the effort to find and repair defects of decreasing size. Electrical holiday detection is most common for coatings used in immersion service. As a result, technical studies of holidays are predominately concerned with coatings in immersion service; there are few studies on the effect of holidays on industrial coatings in atmospheric exposure. Generally, laboratory test panels in most studies are holiday checked and holidays are repaired prior to testing. Of course, since test panels are usually simple geometries and coated under ideal conditions, holidays are less common than on realworld structures. In an unpublished work, 16 the impact of holidays was identified as key to a manufacturing process. However, it was recognized that a holiday test was not a cost-effective inspection technique the impact on production was simply unacceptable. As an alternative, the relationship between number of holidays and coating thickness was developed (Figure 7). This relationship, combined with knowledge of the typical film thickness distribution (i.e., random variation) allowed the adjustment of the process to increase the probability of a holiday-free primer simply by specifying a higher thickness in areas of concern. Alternatively, a reduced surface profile may have achieved the desired result Pass Holiday Test Fail Holiday Test Primer Layer Thickness (mils) Specification Requirement Surface Profile (mils) Figure 6. Holidays as a Function of DFT 16 Unpublished study of M113 Hull painting by Ault, et. al.

9 In another effort, the number of holidays on various sections of bridges was measured and expressed as a quantity per lineal feet of beam edge (Figure 8). 17 Coating thickness was between 10 and 30 mils. All but one of the sections sampled had at least one holiday. No correlation between the number of holidays and film thickness is apparent. Figure 7 suggests that one might specify a minimal primer DFT of at least one mil over the blast profile maximum specified peak-to-valley depth. This corresponds with previously quoted SSPC guidance. 18 Such a thickness should ensure a holiday free primer which can be key to premature coating breakdown. However, Figure 8 also suggests that sufficient primer thickness alone does not ensure a holiday free coating. These two examples illustrate that approaches which work for one process or structure do not always translate to another. Figure 9 shows the data from Figure 8 plotted as a cumulative probability for various beam types. The data is interesting in that it highlights the increased likelihood of holidays on various structure geometries. Bolted geometries contain the most holidays per square foot of surface followed by riveted geometries. More generally, the data demonstrates the effect of complexity on holiday formation. The combination of inspection cost and challenges of obtaining a holiday free coating on complex structures may be cited as reasons that electrical holiday detection is impractical for atmospheric coatings. Since coating failure is likely to begin at defects, it seems logical that holiday detection could be used to determine the quality of an atmospheric coating system. The above discussion shows how electrical holiday detection can be used to differentiate coating quality. Holidays per Square Foot Average DFT - mils rolled with rivets rolled riveted bolted Figure 7. Holiday Density vs. Dry Film Thickness 17 Ault, unpublished presentation at SSPC New England conference, Surface Profile for Anti-Corrosion Paints, Steel Structure Painting Council, 1974.

10 Cumulative Probability 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Holidays per square foot rolled with rivets rolled riveted bolted Figure 8. Holiday Propensity v. Steel Form Sequence of Painting Activities There are a variety of non-technical tradeoffs which can influence the effectiveness of a painting project. For example, sequencing of painting activities can affect the types of materials chosen, effort required to prepare the surface, volume or re-work required and facilities required to complete a project. In the shipbuilding industry, steel is received, cut, and assembled into increasingly larger shapes over a period of time that can exceed a year. Since much shipbuilding is by necessity in a marine coastal environment and shipbuilding activities tend to produce debris the shipbuilder needs to consider the effects of the environment during fabrication and erection. Several alternative strategies for managing surfaces prior to application of the finish coat were observed during a benchmarking study of foreign shipyards. 19 Most shipyards employ some type of pre-construction primer to protect the steel during fabrication. Some shipyards will apply a complete coating system to all but the weld seams as early as possible in production. Others use additional temporary protective coatings during the build process. In addition, some shipyards carry out as much fabrication as possible in areas that are protected from the weather. Other shipyards are continuously touching up damaged edges with pre-construction primer. In some cases, the steel design and tempo of construction activities eliminate much of the concern. Each of the strategies for protecting bare and partially coated steels during construction is designed based on the unique philosophies, needs and capabilities of the individual shipyard. No single strategy is correct and each involves a series of individual tradeoffs. It is important to recognize that the build strategy is the composite of many influences and protective coating application is only one. 19 Foreign Shipyard Coatings Benchmarking Study; an NSRP Surface Preparation and Coatings Panel Project Report. National Shipbuilding Research Program, 2013.

11 Farschon, et al discuss the importance of the sequence of activities on planning a bridge re-painting project. 20 Four projects were reviewed that exemplified how planning the sequence of painting is a vital part of the overall bridge rehabilitation project design. The examples each highlight a different approach to integrating structural steel repairs with protective coating application. A key issue is when in the construction sequence to remove existing coatings, and apply the various components of the coating system. The paper illustrates the importance of a well thought out design scheme in minimizing external impacts, costs, and overall construction time. The lack of a plan can result in an illogical sequence of operations and potential wasted resources. Summary The paper attempts to demonstrate how re-thinking standard requirements may help to increase the value proposition of an industrial coatings project. By challenging industry standard requirements and owner or specifier may be able to increase the value of an industrial coatings project by either reducing the cost or improving the quality. While ultimately a demonstration project on an owners structure will be the best way to prove a value-adding change, there is data in the literature which facilitates the decision-making. The paper presented data from three technical areas: Data presented confirms the industry consensus that abrasive blasting will lead to better corrosion protection than hand tool cleaning prior to coating application. However, the data challenge the notion that higher levels of abrasive blast cleaning result in improve corrosion protection. The coating specifier must understand the cost impact and potential benefits of specifying a particular blast profile. By broadening the range of acceptable profile and/or changing the requirement form a profile depth to another metric (e.g., peak density or coverage of peaks) which may be more cost-effective. Examples show how coating quality can be differentiated using electrical holiday detection. Since coating failure is likely to begin at defects, holiday detection can be effectively used to determine the quality of an atmospheric coating system. There are also non-technical factors which can unnecessarily constrain competition. Specifications which unnecessarily dictate a particular sequence of operations or constructions methods can give some companies a competitive advantage. While this may be increase the likelihood of a successful project it may also reduce competition and increase cost to the owner. The project designer need to consider if requirements that may constrain competition truly add value to the project. 20 Christopher Farschon, Michael Hewins and Charles Moran, Repainting Bridges During Rehabilitation Projects: Sequencing Options, Presented at SSPC 2006, Tampa, FL.