Can In-Process Quality Control Prevent Premature Coating Failures?

For decades we have heard that the incidence of premature coating failure would decline by explicitly requiring the contractor to control the quality of workmanship (via contract document language) using properly trained (and equipped) quality control personnel. In this Case from the F-Files, we’ll take a brief look at five case history failures and assess whether quality control inspection of the work as it proceeded could have prevented the failure from occurring, or whether it would have happened despite the efforts of knowledgeable quality control personnel.

Defining Quality Control

The ISO definition states that quality control is the operational techniques and activities that are used to fulfill requirements for quality.1 This definition could imply that any activity, whether serving the improvement, control, management, or assurance of quality could be a quality control activity. Quality control is a process for maintaining standards and not for creating them. Standards are maintained through a process of selection, measurement, and correction of work, so only the products or services that emerge from the process meet the standards. In simple terms, quality control prevents undesirable changes in the quality of the product or service supplied. The simplest form of quality control is illustrated in Fig. 1. Quality control can be applied to particular products; to processes that create the products; or to the output of the whole organization, by measuring its overall quality performance.

quality control
Fig. 1: The generic control process
Figure and photos courtesy of KTA-Tator

Quality control is often regarded as a post-event activity, that is, a means of detecting whether quality has been achieved and taking action to correct any deficiencies. However, one can control results by installing sensors (e.g., inspection check points) before, during, or after the results are created. It all depends on where you install the sensor, what you measure, and the consequences of failure.1

The Joint Certification Standard for Shop Application of Complex Protective Coating Systems (AISC SPE/SSPC-QP 3 420-10) defines quality control as the inspection of work. Inspection includes but is not limited to confirming that procedures are met; workers are properly qualified; equipment is appropriate and in acceptable working order; and the proper materials are used and are in compliance with inspection criteria.

Let’s take a look at a few case studies to see whether implementation of a quality control program using trained, properly equipped inspectors makes a difference.

Case Study No. 1: Mirror, Mirror…

Background: A contract was awarded to remove and replace the existing coating system on a large riveted structure. The specification required abrasive blast cleaning to achieve a Near-White blast (SSPC-SP 10/NACE 2), followed by two coats of a polyamide epoxy (standard gray) and one coat of polyurethane topcoat. Six months after the contract was completed, corrosion was observed (Fig. 2).

corrosive coating
Fig. 2: Corrosion products on the back sides of the rivets and edges after six months’ service

Cause: Corrosion products remained on the back side of the rivets that were not subjected to direct impact by the abrasive stream during blast cleaning. The coating was also applied from one direction, causing thin areas of coating on the back side of the rivets and the adjacent flat areas of the steel plate. Inadequate attention was given to the coating along the edges.

Avoidance Through Quality Control Inspection? The QC inspector should have carefully examined the “difficult access” areas after surface preparation and application of each coating layer. As a general rule, if the quality control inspector has difficulty accessing the areas, then the coating applicators likely had difficulty as well. Verifying coverage on an abrasive blast-cleaned surface with a gray coating can be challenging. Good lighting and the use of an inspection mirror would likely have revealed the missed areas. The specifier could have selected a contrasting color for the primer and may have required stripe coating in these areas (in accordance with SSPC-Guide 11) to help protect the edges.

Case Study No. 2: The Fix is in, and That’s the Problem!

Background: The project specification required abrasive blast cleaning to achieve a Near-White blast (SSPC-SP 10/NACE 2), and the application of a single coat of an inorganic zinc primer to piping. Surface preparation and coating application were performed in the shop. Once the piping was installed in the field, damaged areas (caused by the installation) were abrasive blast cleaned and touched up with an organic (epoxy) zinc-rich primer. All of the touch-up areas performed well. However, within one year, portions of the piping showed extensive pinpoint rusting and rust-through.

A closer examination of the pipe (Fig. 3) shows one of the rusted areas, with the edge of a repair area also shown (left portion of Fig. 3). As illustrated, the repair area is performing well, but the surrounding area is exhibiting rusting.

Cause: When repairing damaged areas, the blaster failed to start and stop the flow of abrasive from the blast nozzle when moving from one damaged area to another. Instead, the blast nozzle was moved to the next location while the abrasive was still flowing at maximum pressure, which caused considerable damage to the coating. This is apparent in Fig. 3, where a round patch of coating had been effectively removed by the abrasive impact, with the surrounding area nicked by the abrasive.

rusted coating
Fig. 3: Close-up of rusted area, with the edge of a repair area also shown on above

Because the zinc primer is essentially the same color as the steel, the damage went unnoticed until the electrolyte (water) contacted the surface and caused the formation of corrosion.

Avoidance Through Quality Control Inspection? Many would point to a misplaced repair procedure as the culprit in this case; and perhaps, even with diligence, the overblast damage may have been unavoidable. However, during the start-up of the repair procedures, the QC inspector should have observed the abrasive blast cleaning operations, recognized the potential for overblast damage, and discussed the issue with the owner/specifier before work continued. The owner and inspector could have discussed alternative methods of preparation. Anticipating potential problems and proposing resolutions before a widespread problem occurs are intangible values that quality control inspection can bring to a project.

Case Study No. 3: You Know What They Say: Dry Heat Is More Comfortable

Background: The project specification required abrasive blast cleaning to achieve an SSPC-SP 10/NACE 2 Near-White blast and the application of an inorganic zinc primer to structural steel components in the fabrication shop. Application of the intermediate coat was also performed in the shop, while the topcoat was scheduled for application in the field after erection and bolting of the steel. The work was done in the winter, and the shop was heated. The fabricator’s quality control specialist kept documentation revealing that the shop coating had conformed to the thickness and recoat times recommended by the coating manufacturer’s technical representative, who visited the shop during coating application. The steel was loaded onto trucks and shipped to the site. When the coated steel arrived at the construction site, spontaneous cracking of the coating along the fillet weld (where the web and flange are joined) was discovered (Fig. 4). Figure 5 illustrates the spontaneous cracking and lifting along the fillet, and the poor adhesion of the coating system on the top of the bottom flange. Examination of a disbonded coating chip revealed the presence of zinc primer on the back side of the chip and on the steel surface, indicating that the location of break was cohesive within the zinc primer.

cracking coating failure
Fig. 4: Spontaneous cracking of the coating along the fillet weld
adhesion coating failure
Fig. 5: Poor adhesion of the coating on the top of the bottom flange

Cause: Ethyl silicate inorganic zinc-rich primers require moisture to cure. In this case, insufficient time was allowed before the application of the epoxy midcoat. Once the epoxy was applied, no more moisture could react with the primer because the epoxy sealed off the primer. The zinc primer remained in a dry but uncured (and weakened state). The solvents from the epoxy midcoat penetrated the uncured primer, and the contractive curing stresses imparted by the epoxy caused the zinc primer to cohesively split. Because a web and flange are adjacent to one another, the thickness of the epoxy was slightly higher along the fillet weld area. The higher thickness exacerbated the problem and resulted in the cracking and detachment. When other areas were evaluated, it became evident that the entire system was at risk for failure.

Avoidance Through Quality Control Inspection? Inorganic zinc-rich primers dry very quickly (especially in a heated environment); however, they may not cure for many hours or even days if the humidity is too low within the prevailing environment. The key is to verify that temperature and humidity (listed on the product data sheets) are present in the shop before application and to verify the cure has been achieved, rather than relying on cure time tables provided by the coating manufacturer, or assuming that drying and curing are synonomous. Quality control inspection by the fabricator should have included a curing test. In fact, there is one specifically designed for the primer in this case study (ASTM D4752, Measuring MEK Resistance of Ethyl Silicate (inorganic) Zinc-Rich Primers by Solvent Rub). Once a resistance rating of “4 or 5” is achieved (after 50 double rubs), the zinc-rich primer can be considered cured and ready for recoating. Some manufacturers rely on pencil hardness data instead of solvent resistance to assess cure. Either way, a competent QC Inspector knows how specific coating types cure, the conditions necessary for the reactions to occur, and the tests available to verify coating film properties before applying the next coating.

Case Study No. 4: A Picture’s Worth Thousands of $$$

Background: The project specification required abrasive blast cleaning to achieve a Commercial Blast (SSPC-SP 6/NACE No. 3) and the application of a single coat of alkyd primer in the joist fabrication shop. The joists were shipped to the project site, where they were stored outdoors (on the ground) for six months. Corrosion was visible within six months (Fig. 6).

Cause: SSPC-SP 6/NACE No. 3 requires removal of all mill scale. The surfaces may have staining from mill scale (provided it does not exceed 33% of each 9 square inches). In this case, the “pock marks” in Fig. 6 clearly indicate that mill scale was left on the surface and coated over. The “hollow” areas represent those locations where the mill scale was removed, while the surrounding areas contain mill scale. The areas containing mill scale exhibit corrosion products. In a mild environment (and with the proper thickness), this system should have lasted longer than six months. However, the application of a thin (3- to 5-mil) film alkyd primer combined with damp storage conditions led to water permeation of the alkyd. The result was the formation of a corrosion cell at the mill scale/steel interface. Mill scale is cathodic to steel, which means that the base steel becomes the anode in the corrosion cell and begins to deplete, generating the corrosion products. Ironically, had the joists been stored indoors (or installed upon receipt), the lack of quality may have never been revealed, because it is unlikely that corrosion would have occurred due to the lack of electrolyte.

Avoidance Through Quality Control Inspection? Careful visual inspection of the steel surfaces by the quality control inspector after surface preparation (including the use of SSPC-VIS 1) would have revealed the presence of mill scale, which is not permitted by the specified cleanliness standard. That is, quality control personnel need to know industry standards and need to use tools (in this case, visual guides) for help in making intelligent decisions. While additional surface preparation before application of the primer would have required additional labor and more abrasive, it would have been done at a significantly lower cost than the cumulative costs associated with the failure investigation, transportation of the joists back to the fabrication shop (and then back to the project site once the rework was done), the material and labor costs associated with re-application of the primer, and potential for liquidated damages due to project schedule delays.

steel corrosion
Fig. 6: Corrosion of steel beneath alkyd primer evident within six months

Case Study No. 5: Hey! I Followed the Spec; It Wasn’t My Fault.

Background: The underside of a viaduct containing an aged lead alkyd coating was brush-off abrasive blast cleaned to remove loosely adhering corrosion and paint (SSPCSP 7/NACE No. 4), followed by the application of an epoxy mastic overcoat. Figure 7 illustrates the condition of the coating prior to abrasive blast cleaning. Figure 8 illustrates lifting of the old alkyd by the epoxy mastic overcoat. The number “10” written on the coating in Fig. 8 is in an area where the epoxy mastic was applied directly to the steel, rather than the aged lead alkyd. Directly beneath that area is an area where the mastic had lifted the alkyd, and was removed by scraping during the failure investigation. The area beneath the hand in the same photo represents epoxy mastic applied over the aged lead alkyd. This area was not probed during the investigation.

brush off blast cleaning
Fig. 7: Condition of the coating on the underside of the viaduct before brush-off blast cleaning

Cause: The aged lead alkyd coating was in poor condition, as illustrated by Fig. 8. While brush-off abrasive blast cleaning removed the loosely adhering materials, the impact of the abrasive on the “intact” alkyd weakened (fractured) it, but did not affect it enough to consider it “loose” by the dull putty knife test. Application of the epoxy mastic imparted curing stresses that weakened the cohesive strength of the aged lead alkyd, causing it to lift and disbond from the surface.

quality control
Fig. 8: Area where epoxy mastic was applied directly to the steel rather than the aged alkyd

Avoidance Through Quality Control Inspection? Because the QC inspector does not have the authority to change the specification, this project was doomed from the minute the work was awarded. Even though the QC inspector may have questioned the specification, it is doubtful that that owner would have altered the spec unless the fracturing of the aged lead alkyd had been visible to the unaided eye and the inspector had informed the owner of the damaged coating. Inspection personnel cannot use magnification (according to the SSPC Surface Preparation Standards).

So while it appears that controlling quality as the work is performed reduces the opportunity for coating failure, quality control cannot be a substitute for a well-written specification, quality coating materials, and quality workmanship.

BCorbett3William D. Corbett is the Vice President and Professional Services Group Manager for KTA-Tator, Inc. He holds an A.D. in Business Administration from Robert Morris University and has been employed by KTA for over 33 years. Mr. Corbett is an SSPC Certified Protective Coatings Specialist, an SSPC Level 3 Certified Protective Coatings Inspector, an SSPC Level 2 Certified Bridge Coatings Inspector and a NACE International Level 3 Certified Coatings Inspector. He was the co-recipient of the SSPC 1992 Outstanding Publication Award, co-recipient of the 2001 JPCL Editor’s Award, received SSPC’s Coatings Education Award in 2006, and the SSPC 2011 John D. Keane Award of Merit. He is the author of the first, second and third editions of the KTA publication, Using Coatings Inspection Instruments. He also authored Chapter 8 of the SSPC Inspection of Coatings and Linings Handbook and co-authored Chapter 6, “Inspection” of the Steel Structures Painting Manual, Volume 1, Good Painting Practice. Mr. Corbett is a member of the ASTM International and SSPC: The Society for Protective Coatings, where he is Chair of the Education Committee and the Dry Film Thickness Committee.