SSPC-PA2 – Is it a Quality Control or a Quality Assurance Standard?

The Author wants to commence this paper by stating that he believes SSPC-PA2 to be an excellent standard. The author also wants to thank those who participated in the standard’s first development in 1973 and all who have participated in the updates over the years, with the latest update taking place in 2022. PA2 is an exceptionally good standard; however, it is often misrepresented in specifications and frequently misused in the field. SSPC-PA2 now falls under the umbrella of AMPP (Association of Materials Protection and Performance) which is the new entity created by the merger of NACE and SSPC a few years ago. This paper addresses the SSPC-PA2 Standard; however, the discussion points are relevant to SSPC-PA9 which concerns non-metallic substrates including concrete, gypsum, wood, and so on. SSPC-PA2 is a more extensively specified standard, which is why the author concentrated on PA2.

Despite the document’s quality, the author has always felt it was more appropriate for use as a Quality Assurance (QA) document than for Quality Control (QC) purposes. Defining the terms of Quality Control (QC) versus Quality Assurance (QA) with accuracy is vital. The AMPP definitions of the two terms are as follows:

Quality Control - Manages the quality of work performed to meet requirements of the project specification.

• Product-based approach

• Focus on the detection of defects.

• Performed throughout the project.

• The contractor’s responsibility

Quality Assurance - Verifies/Audits that the quality of work performed is what was reported by the Contractor’s QC personnel.

• Process-based approach.

• Focus on the prevention of defects.

• Performed at key stages.

• The owner’s responsibility

Quality Control is always the responsibility of the contractor and is a full-time effort that requires trained QC personnel holding an AMPP Coating Inspection credential. The minimum level of training allowed for a QC inspector may vary depending on the demands of the specification. A Basic Coating Inspector (Formerly referred to as a Level 1) can be used for QC inspection providing they are under the direct supervision of a AMPP Certified Coating Inspector (Formerly referred to as a Level 2) or a Senior Coatings Inspector (Formerly referred to as a Level 3).

Quality Assurance inspectors should be a minimum AMPP Certified Coating Inspector or a Senior Coatings Inspector. A Basic Coating Inspector could be used for QA inspections providing they have enough experience. There are Basic Coating Inspectors who have been in the industry for many years and for assorted reasons chose not to pursue further certification levels. Basic Coating Inspectors who have less than 5 years of steady inspection or other industry experience may not be suitable for a QA inspector role.

For complex projects needing timely verification, QA inspection can be a demanding full-time job. Another option for QA inspection is “Hold point inspections.” Hold Point Inspection is a QA method where inspections by QA are performed at crucial project stages—post-pre-cleaning, surface prep, primer, intermediate and final coat application, and final testing. Hold point inspections can be valuable if managed effectively without adding undue cost to a project.

The author will justify these assertions on the use of SSPC-PA2 and SSPC-PA9 in this paper. This paper aims to encourage the proper use of the standards in the field and to encourage proper wording in specifications. Proper designation of SSPC-PA2 and SSPC-PA9 will lead to better long-term infrastructure and project performance, decreasing protective coating, lining failures, and increasing the life cycle of assets.

Background

Effective corrosion control and preventing premature coating failure on any project depends on proper surface preparation and application of protective coatings and linings. Protective coatings and linings are meticulously engineered, with precise design parameters dictating film thickness on the protected surface. When the proper products are selected, and applied within their design parameters, the result of proper selection, preparation and application is extended life cycle protection for the asset they are applied to. Consistent and proper dry film thickness (DFT), as designed, is key to corrosion protection and failure avoidance.

People unfamiliar with coatings and linings may not know that most bridges, water tanks, power plants, and other infrastructure rely on thin protective coatings (8–10 mils) for exterior protection. In case you are unfamiliar, a mil equals 8-10 thousandths of an inch. One mil is similar to the thickness of one layer of saran wrap. These products are designed to provide maximum corrosion protection even at exceptionally thin DFT. For years, epoxy linings (8–12 mils thick) shielded potable water storage tank interiors constantly submerged and exposed to condensation. There are still municipalities specifying thin film systems on water storage tanks, however, over the past decade specifiers have begun to understand that increasing the DFT of interior linings has a consequent effect on extending the asset’s lifespan. Adoption of 100% solids technology has boosted DFT in tank linings to 20–30 mils, extending lifespans to 20–30 years or more. We do not yet know exactly how long they will perform; this is an ongoing study. I can assure you that I have been in tanks that have had a high film build protective lining applied that are now exceeding 30 years in service and still going strong. Proper DFT is crucial for extending the lifespan of protective coatings, linings, and the asset itself. Note that DFT values should be specified as ranges, not single values, by both the manufacturer and specifier. The range should also be reasonable and realistic. I have long contended that a 1-2 mil range is remarkably difficult for even the most skilled applicators to reach and often unjust given human, not robotic, application under challenging conditions. Unrealistic DFT ranges often lead to disagreements between quality control, quality assurance, contractors, applicators and building owners. Coating or lining manufacturers bear significant responsibility for problems stemming from unrealistic DFT ranges. Engineers and owners depend heavily on the manufacturer’s data for the DFT range.

The Process

The known link between proper DFT and extended asset life necessitates correct application. Quality Control and Quality Assurance are crucial to confirm that surface prep and protective coating/lining application completely meet specifications and manufacturer requirements. Here’s where things start to get misinterpreted. As Sr Principal Consultant at Corrosion Probe Inc (CPI), the author often revises and updates specifications from engineering firms and clients (municipal and private) across North America. What we consistently see in specifications is that when QC language is included in the specifications (you may be surprised by how many specifications do not have any QC language at all, or it is convoluted or confused with Quality Assurance language), the specifications simply state that DFT measurements shall be performed as per SSPC-PA2 for metallic substrates and SSPC-PA9 for non-metallic substrates such as concrete, gypsum, wood etc. Here, the difficulties of defining standards SSPC-PA2 and SSPC-PA9 become clear. SSPC-PA2 and SSPC-PA9 provide valuable details for both QC & QA regarding how to verify that the gage is measuring properly (gage accuracy should be verified multiple times during a shift), and how to investigate areas that are found deficient, however, when it comes to measuring the DFT, the number of DFT readings the standard requires does not provide enough data points to accurately assess the overall DFT for Quality Control purposes. SSPC-PA2 addresses two distinct types of DFT gages, Type 1 Magnetic Pull Off Gages and Type 2 Electronic Gages.

We only use Type 1 gages today when intrinsic safety is needed (no power source) or for quick checks. This article will not cover the far more complex accuracy verification process required for Type 1 Gages. Type 1 DFT gages lack memory, preventing data storage. With Type 1 gages each reading must be recorded manually.

Type 2 Electronic Gages, which feature higher accuracy, easier verification, and massive data storage capacity (single or batch readings), are the leading DFT measurement tools. Type 2 gages commonly include an SSPC-PA2 setting. Measurement of SSPC-PA9 DFT is done using an ultrasonic gage. This information applies to both QC and QA inspectors. The problems with PA2 and PA9 are most apparent in the number of DFT readings taken. The following DFT data points information is sourced from SSPC-PA2 (PA9 is similar, although the spot measurement area size varies; our analysis focuses on PA2).

8.1- Number of Measurements - Repeated gage readings, even at points close together, often differ due to small irregularities of the coatings and the substrate. Therefore, a minimum of three (3) gage readings shall be made for each spot measurement of the coating. For each new gage reading, move the probe to a new location within a 1.5 inches (4 cm) circle defining the spot. Discard any unusually high or low gage readings that are not repeated consistently. The average of the acceptable gage readings (Min 3 readings) is the spot measurement.

8.2 – Unless otherwise specified in the procurement documents (project specification), an area measurement is obtained by taking five (5) separate spot measurements (average of the gage readings described in section 8.1) randomly spaced throughout each 100 ft² (10m²) area to be measured and representative of the coated surface. The five spot measurements shall be made for each 100 ft² (10m²) area as follows:

8.2.1- For areas of coating not exceeding 300 ft² (30m²) arbitrarily select and measure each 100 ft² (10m²)

8.2.2- For areas of coating greater than 300 ft² (30m²) and not exceeding 1000 ft² (100m²), arbitrarily select and measure three 100 ft² (10m²) areas.

8.2.3- For areas of coating exceeding 1000 ft² (100m²), arbitrarily select and measure the first 1000 ft² (100m²) as stated in Section 8.2.2. For each additional 1000 ft² (100m²) coated area (or increment thereof) arbitrarily select and measure one additional 100 ft² (10m²) area.

Note that “arbitrarily select” is not proper Quality Control terminology before we proceed to a PA2 structural measurement example. Quality Control is an extremely critical process and nothing about the process should be considered “arbitrary.”

To support this claim, consider a square carbon steel tank interior: 50 ft long, 50 ft wide, and 20 ft deep.

50’ x 20’ = 1000 ft²

4 walls x 1000 ft² = 4000 ft²

Tank Ceiling- 50’x 50’ = 2500 ft²

Tank Floor- 50’x 50’ = 2500 ft²

Total Square Footage of the coated tank = 9000 sq ft.

The SSPC-PA2 standard indicates the following DFT reading requirements:

For the first 1000 ft² you arbitrarily select and measure three separate 100 ft² (10m²) areas. This results in 15 spot measurements (a spot is the average of 3 gage readings in a 1.5 in circle), 5 spots per each 100 ft² (10m²) area.

For each additional 1000 ft² of coated surface arbitrarily select one 100 ft² (10m²) area and take 5 spot measurements. This means 8 more arbitrarily selected 100 ft² (10m²) areas would result in 40 spot measurements (8000 ft² x 5 spots per 1000 ft²)

Here is a summary of the final DFT measurement totals:

First 1000 ft² = 15 Spot Measurements (5 spots in 3 arbitrarily selected 100 ft² (10m²) areas) Remaining 8000 ft² area- 40 Spot Measurements (5 spots per each additional 1000 ft²)

Total Spot Measurements= 55 Spot Measurements

The data comprises 165 separate gage readings (55 locations, 3 readings per location).

Here, the inadequacy of PA2 as a Quality Control Standard becomes glaringly apparent. Picture the size of the 50’ x 50’ x 20’ deep tank and the surface area involved. Consider whether 55 data points suffice to ensure complete surface coverage and protection. In my time as a Quality Control inspector, this author can confirm that 55 spot readings are insufficient. 9000 ft² of surface area requires thousands of gage readings to ensure conformity, not 165 gage readings.

The Quality Assurance role, as previously discussed, is to audit/verify the accuracy and completeness of the QC process. Quality Control’s job is to minimize the chance that QA’s random checks will uncover serious flaws. The owner also indirectly bears the costs of both QC and QA, it should be mentioned. They pay for QC through the contractor’s contract bid (providing QC is properly detailed in the project specs) and QA by either performing the QA work themselves with properly trained and certified coating inspectors, or by utilizing independent QA inspectors retained by the owner or the owner’s representative (i.e. Project Engineer).

With SSPC-PA2 only requiring 55 spot measurements from the contractor’s QC staff for our 9000 ft² tank, the potential problems are evident. 55 spot measurements are not nearly enough to ensure proper protection against corrosion.

Standard Tolerance

A further significant misunderstanding of SSPC-PA2 (and PA9) relates to the standard’s tolerance. Five (5) restriction/tolerance levels are defined in SSPC-PA2. If unspecified, the level defaults to Level 3. We will use Level 3 as a method of explaining the allowable tolerance. See Table 1 diagram taken directly from the SSPC-PA2 Standard.

Table 1 - Coating Thickness Restriction Levels

SSPC-PA2 Level 3 (default) allows DFT values within 80% of the minimum and 120% of the maximum allowable values. There is a widespread misunderstanding of this tolerance among applicators and specifiers. Uncertainty stems from the 80%-120% tolerance rule. The 80%-120% tolerance rule is often wrongly assumed to apply to all measurements. Specifically, the 80%-120% tolerance is only for individual spot measurements, not the overall DFT average. Table 1 shows that individual gage readings are not restricted, as long as their average (the spot measurement) falls within 80-120% of the DFT range. However, the average of all the spots’ DFT values must fall within the specified range. Final DFT averages are not subject to the 80%-120% tolerance rule. This is commonly misunderstood. Although SSPC-PA9, unlike the example PA2, only has four tolerance levels and different percentages, it functions similarly. Incomplete understanding of tolerance levels can result in mistakes leading to shorter service life and coating or lining failure. To illustrate, let us consider an 8-10 mil DFT coating range. Using the default Level 3 tolerance, this means that any spot measurement average could be as low as 6.4 mils and as high as 12 mils providing the overall average of all spots measured falls within the specified 8-10 mil DFT range. This is why it is extremely important that manufacturers provide a realistic DFT range to avoid QC/QA conflicts in the field.

Conclusions

The effectiveness of protective coatings and linings depends on precise installation to the correct thickness and appropriate consideration of their designed service environment (immersion, soil, chemicals, UV). This includes solvent based coatings and linings, 100% solids coatings and linings, water-based coatings, and all other types of Protective Coatings. Coating applications beyond the recommended dry film thickness can lead to trapped solvents (solvent-based coatings), impaired or incomplete curing (solvent and water-based coatings), and compromised film formation. In addition, over-application is particularly problematic for several products, such as Inorganic and Organic Zinc Rich Primers, Novolac Epoxy, Acrylic Aliphatic Polyurethanes, and specific Vinyl Ester formulations. Inadequate DFT, lower than the manufacturer’s recommendation, can cause pinpoint corrosion (if blast profile peaks are not sufficiently protected), insufficient barrier coat thickness (leading to higher permeability), poor adhesion, poor film quality, and a less appealing result. Applying any engineered materials outside of the parameters of the materials design, either high or low leaves the owner at risk of a premature coating or lining failure or at the very least a product that will not perform to its intended design.

While SSPC-PA2 is an excellent standard, as noted earlier, its real-world application is flawed. This standard works well for Quality Assurance but falls short as a Quality Control standard. When referencing SSPC-PA2 or SSPC-PA9 in Quality Control specifications, strong wording is needed on how inspectors should use these standards. Failing to set the parameters of using SSPC-PA2 or SSPC-PA9 properly for Quality Control leaves the owner at risk.

The author fully understands that this paper may be viewed by some as a negative commentary on SSPC-PA2 and PA9. This includes AMPP itself. That is certainly not the intent of this paper. As a long-time supporter of both SSPC and NACE and now AMPP over my career, to which the author has provided countless hours of teaching and volunteering, nothing could be further from the truth. The intent of this paper is to bring awareness to the significant misunderstanding and often misuse of an otherwise excellent standard. As an industry we must be willing to adapt and change when we realize something is not delivering the intended results. Such is the case with PA2 and PA9. Both are written in a way to be utilized as Quality Assurance standards not Quality Control. The author believes they have expressed why this is the case. These efforts will go a long way in further protecting owners from premature coating failures and extending the lifecycle of their assets.

Murray Heywood, Sr Principal Consultant, Corrosion Probe Inc.

SSPC- PCS – Protective Coating Specialist SSPC Master Coating Inspector #33 NACE Level 3 Certified #11175

MPI Architectural Coating Specialist #0389 Cell- 905-260-8246

Email - heywoodm@cpiengineering.com