Priming of carbon steel plates, beams, angle irons, and other shapes before their fabrication into parts of industrial and marine structures has progressed from its earliest stages of fabrication to the current block stage, or module process. The earliest practices used the full thickness of both organic and inorganic primers intended to be the first coat in multi-coat systems. It was recognized that these full coat primers impeded the progress of welding and led to porosities in the welds, compromising the integrity of the entire coating system. Extensive damage during the fabrication process led to a need for quicker processes with fewer intercoat adhesion problems.
As seen in the November 2011 Edition of the Journal of Protective Coatings & Linings (JPCL) and on Paintsquare.com
The coatings industry met this need with more efficient inorganic zinc primers. The industry adopted thinner films and films with less zinc content to meet the production requirements of the fabricators—predominantly shipyards. While some organic pre-construction primers (mainly waterborne inhibitive epoxies) have been used, they did not match either the production efficiencies or the protection of the carbon steel afforded by inorganic zinc pre-construction primers (PCPs) during the fabrication process, which often extends beyond a year. The major problems were extensive burn back from the weld area, fabrication damage, and noxious fumes during welding and cutting processes.
Inorganic Zinc Pre-construction Primers (PCPs)
Inorganic zinc pre-construction primers (PCPs) are a special class of inorganic zinc primers. Depending on the manufacturer, permanent inorganic zinc primers are formulated with zinc pigment content ranging from 45% to 92% and an end use requirement of 2–4 mils’ (50–100 μm) dry film thickness (DFT). All inorganic zinc PCPs have a drastically reduced zinc pigment content with an end use requirement of 0.6–0.8 mils’ (15–20 μm) DFT. PCPs are designed to withstand the welding processes and handling damages during the fabrication process of blocks or modules before incorporation into the permanent finished structure. PCPs provide corrosion protection, but only from their application until the fabricated components are welded into modules. Application of the permanent protective coating system varies according to the shipyard’s individual production process. It may involve application of just the first coat of the permanent protective coating system just before the module stage or application of the first and second coats. The finish coat of the permanent system is rarely applied before the module stage.
Zinc content of PCPs is typically in the 28–48% range. Zinc content is chosen based on a tradeoff between welding issues (favors lower zinc content) and corrosion protection during storage (favors higher zinc content). Other factors, such as cost and usability, will also contribute to the final formulation. A common solution in the lower cost versions is to replace some of the zinc with inhibitors like zinc phosphate, molybdates, chromates, etc. One of the first of the inorganic PCPs recommended for immersion service of more than 15 years without sweep blasting had zinc oxide, vitreous silica, and kaolin in its formula.
PCP selection is driven by welding as well as coating issues. The primer can have an effect on welding speeds and quality. The composition of the primer can have health and safety implications for the welding process. A 1973 National Shipbuilding Research Program (NSRP) report discussed these issues.1
Shipyards in the Far East and Europe often use weldable zinc silicate PCPs in the block construction. Undamaged primers, after block erection, are not removed but given a secondary surface preparation designed to permit good adhesion of the full protective system. Areas of damage, through welding or erection, are either blasted to Sa 2½ during block construction or power tool cleaned to St 3 after erection of the blocks, followed by the full coating system.
This article highlights some of the research work that has been carried out to support this practice and describes some “standards” in use around the world.
Global PCP Standards
A significant challenge in the industry is that there is no standard for PCP material. A variety of coating materials have been marketed as PCPs. In the absence of a global standard, classification societies such as ABS, DNV, Lloyds, etc., issue type approval certificates for PCPs. These certificates indicate that the materials meet the class society’s requirements for PCPs but do not establish a standard or meet a global standard. Competition among the societies for dominance in a particular market creates the potential for less stringent standards.
The debate over whether to retain PCPs is not new. NSRP has sponsored a number of studies over the past 30 years that have looked at the issue. Several commercial shipyards outside the U.S. have recently performed testing in support of the IMO PSPC regulations. The following is a brief review of selected studies.
A 1985 NSRP report assessed what was then called the “Japanese methodology” to see if it could provide adequate corrosion protection to their ships, given a 20-year life cycle of a ship.2 A key consideration of this methodology was never removing the PCP to bare metal. The investigators visited four different ships, all at a different period in their life cycles. The ships included a container ship at one-year service life, a large tanker at six years into its life, a car carrier at eight years of service life, and a bulk cargo ship with 14 years of service. At each inspection, assessments were made (whenever accessible) of coating condition on the freeboard, deckhouse, underwater hull, internal tanks, and engine room coatings. Finally, chief chemists of two large Japanese coating manufacturers were interviewed. The general message from this report was that the Japanese coating systems are standard, simple, and designed for maintainability at predictable intervals with shipyard production in mind. As such, they are “good enough” to get a ship 20 years with “adequate” corrosion protection. Other salient points of the report include:
- Assembly blocks are completely outfitted before attachment; therefore, dust must be kept at a bare minimum. Open blasting is virtually eliminated in construction; heavy emphasis is placed on secondary surface preparation, typically disc sanding.
- The Japanese shipyards utilized standard coating systems (for very specific reasons), unlike in the U.S., where several coating types of varying sophistication are employed. For example, PCPs consist of low-film build (0.6 mils’ DFT typically), alkali silicate primers with zinc dust, allowing for fast cutting and weld-thru times. Higher build, zinc-free PCPs (1.5–2.0 mils’ DFT) can lead to slower cutting and welding, unacceptable fumes, and weld porosity. The standard ballast tank coating is coal tar epoxy. The standard exterior hull coating above the boot-top (freeboard) consists of chlorinated rubber, which is recoated very easily every four years. The standard underwater hull system is coal tar epoxy, followed by a vinyl tar tie coat to an ablative anti-fouling layer.
- The four ships all exhibited varying signs of coating breakdown and wear, based on the age and service area of the coating. The coatings selected and the quick turn-around maintenance (never removing the PCP to bare metal) provided “adequate” corrosion protection for the ships’ design life of 20 years.
- Other specific points are that the thin film PCPs used above the waterline did not provide adequate corrosion protection from undercutting, as evidenced after one year. There was a heavy dependence on secondary surface preparation, followed by touch-up with organic zinc coatings.
NSRP sponsored two laboratory test programs during the same time as the aforementioned visit to the Japanese shipyard. One study involved topcoating a number of inorganic zinc primers with various epoxy topcoats and subjecting them to various immersion performance tests to see if delamination and blistering occurred.3 In a second study, four performance tests were carried out on six coating systems (coal tar epoxy, polyamide epoxy, inorganic zinc, chlorinated rubber, vinyl, and bleached tar) applied to retained and removed PCP.4 The tests included: (a) six-month, 150 F salt water immersion; (b) three 17-day cycles each consisting of 14 days’ immersion at 80 psi and three days of atmospheric exposure; (c) two-month alternating cycles of UV, heat, and immersion; and (d) 2000-hour cyclic salt fog testing per ASTM B-117. The laboratory testing did not lead to any conclusive position on the retention of PCP, but both studies contained detailed observations on the effect of various topcoats and zinc loadings in the primer.
NSRP also sponsored two tests of PCP retention in simulated ballast tanks. A study completed in 1982 included retained PCP as part of a project looking at various coating systems with and without cathodic protection.5 After one year, no depletion of the inorganic zinc PCP was observed, along with minimal loss of zinc anode. The author concluded that this system, based on the mockup test, probably could last 20 years without replacement of the zinc anodes. The other systems had diminished performance with the soft coatings performing so poorly that they were removed from the testing phase a few months into the test.
Another project involved a series of four simulated ballast tank assemblies, which were fabricated at NASSCO (San Diego, CA) in November 1998. Each tank was blasted and coated with various PCPs (a weldable control, a waterborne zinc PCP, and two solvent-borne PCPs). The tanks were fabricated and allowed to weather 60 days. Various topcoats were applied after secondary surface preparation (SSPC-SP 1, SP 3, and SP 11 of welds). Two control tank compartments were included in the study; the control tanks had the original PCP blasted off to SSPC-SP 10 and, in one, the NAVSEA high-solids system was applied. The other control tank had MIL-P-24441 type IV applied.
Based on the overall performance of the coating systems tested throughout the four-year cycle, the author concluded, “The use of preconstruction primer in conjunction with standard tank lining systems did not degrade the overall performance of the lining system.”6
In 2005, Hyundai Industrial Research Institute reported on the performance of two mock-up blocks using Korean shipbuilding procedures.7 After fabrication, a one-month weathering period was observed. One mock-up was sweep blasted, and the other was fully blasted. Both assemblies were coated with standard ballast tank epoxy topcoats. Tensile adhesion tests on both tanks met NORSOK requirements. This data, along with cathodic disbondment testing per ASTM G8,8 was used to conclude that retention of PCP is a viable option for process improvement and cost associated with shipbuilding and bridge building.
The nearest thing to a standard is the International Maritime Organization (IMO) Resolution (MSC215(82)), Performance Standard for Protective Coatings for Dedicated Seawater Ballast Tanks in all Types of Ships And Double-side Skin Spaces of Bulk Carriers (PSPC). This resolution, which became mandatory through an amendment of the SOLAS convention, applies to ships contracted on or after July 1, 2008. It specifies inhibitor-free zinc silicate shop primer or equivalent. (At least one zinc-free shop primer has been approved.) Equivalence is not defined; therefore, the determination of equivalence must be proved by the manufacturer of the PCP and submitted to the classification society, which will issue a Type Approval Certificate. However, it should be noted that this is only recognized by the Class issuing the certificate; it is not universally approved. This Resolution is based on specifications and requirements intended to provide a target useful for a coating life of 15 years for the entire coating system. The actual useful life will vary, depending on variables including actual service conditions.
The specific requirements of the IMO PSPC Resolution relating to PCPs include:
- Shop-primer is defined as “the prefabrication primer coating applied to steel plates, often in automatic plants (and before the first coat of a coating system).”
- “The shop primer shall be zinc containing, inhibitor free zinc silicate based, or equivalent.”
- “Shop primer compatibility with main coating system shall be confirmed by independent testing of the complete system carried out for the coating manufacturer.”
- The complete coating system comprising epoxy based main coating and shop primer shall have passed a pre-qualification certified by test procedures in annex 1. The annex describes a 180-day test of panels installed in a simulated ballast tank with wave movement.
- If the complete coating system, including PCP, has been approved, “the retained shop primer shall be cleaned by sweep blasting, high-pressure water washing, or equivalent method.”
The Japan Ship Technology Research Association (JSTRA) published its Standard for the Preparation of Steel Substrates for PSPC-2008 (SPSS for PSPC). This photographic collection of surface preparation examples is intended as a reference when implementing the PSPC. These photographs may also serve as a reference when applying the Performance Standard to void spaces as well as cargo oil tanks developed or to be developed by the IMO.
ISO 8501-1:1998/Suppl:1994 provides photographic examples of grades Sa 2½ and St 3 surface preparation, but only for rusted flat steel surfaces without a shop primer. It does not provide photographic examples of typical areas after block assembly or at the erection stage. Several coatings manufacturers have their own process standards for preparing aged PCP before coating application.
Between priming and fabricating steel, the PCP is likely to be contaminated or damaged from handling, outdoor storage, and/or welding. While the PCP does not have to be removed, a surface preparation step must be accomplished before applying the final coating system. This step, often called “secondary surface preparation,” may consist of the methods, or a combination of the methods, discussed below.
SSPC-SP 1, Solvent Cleaning, describes a method for removal of oil, grease, dirt, soil, salts, and contaminants by cleaning with solvent, vapor, alkali, emulsion, or steam. This procedure is commonly performed on a spot or as-needed basis, though it can be applied to an entire surface regardless of initial condition. It can be used as a stand-alone cleaning method, but it is also recommended before performing any of the following surface preparation techniques.
Power Tool Cleaning
Welds and damaged primer should be mechanically cleaned to remove surface oxides and restore a profile before applying the main coating system. In many cases, power tool cleaning is the most cost-effective way to prepare these sections. There are several specifications for power tool cleaning including the following.
- SSPC-SP 3, Power Tool Cleaning, covers removal of loose rust, mill scale, and paint to the degree specified by power tool chipping, descaling, sanding, wire brushing, and grinding.
- SSPC-SP 11, Power Tool Cleaning to Bare Metal, covers complete removal of all rust, scale, and paint by power tools, with resultant surface profile.
- SSPC-SP 15, Industrial Grade Power Tool Cleaning, is the specification between SP 3 and SP 11 and calls for removing all rust and paint but allows for staining; requires a minimum 1 mil (25 μm) profile.
Pressure washing of a surface that has a PCP is an economical way to address the entire surface. By pressure washing the entire surface, non-visible contaminants should be removed. SSPC-SP 12/NACE No. 5, Surface Preparation and Cleaning of Metals by Waterjetting Prior to Coating, defines four degrees of cleaning for visible contaminants (similar to SP 5, 6, 7, and 10) and three levels of flash rust. SSPC-SP 12/NACE No. 5 also describes three levels of non-visible surface cleanliness for non-visible soluble salt contamination. For preparation of surfaces that have PCPs, low-pressure water cleaning should be used to achieve a WJ-4 condition (Light Cleaning). This involves using water at pressures less than 34 MPa (5,000 psig) to create a surface finish that, when viewed without magnification, is free of all visible oil, grease, dirt, dust, loose mill scale, loose rust, and loose coating. After a surface is pressure washed, welds and damaged primer will require power tool cleaning or abrasive blasting. The mechanical surface preparation would remove any rusting (or flash rusting) that might occur. A level of acceptable soluble salt contamination should be specified.
Abrasive blasting can be used to sweep blast intact primer, as well as prepare welds and damaged areas. Sweep blasting is performed in accordance with SSPC-SP 7/NACE No. 4, Brush-Off Blast Cleaning. The standard requires blast cleaning of the entire surface, except tightly adhering residues of mill scale, rust, and coatings, while uniformly roughening the surface. All pre-construction primed surfaces may be sweep blasted. Sweep blasting of PCPs can be challenging because the primer color is similar to that of abrasive blasted steel. It can be difficult for blasters who are used to near-white metal blast to adjust to the lesser level of surface preparation. However, once workers are trained to sweep blast, they can roughly double the production rate and reduce the amount of abrasive consumed compared to a near-white metal blast.
Welds and damaged areas require a higher degree of surface preparation to remove any oxides and create a surface profile. These areas should be prepared to one of the following specifications.
- SSPC-SP 6/NACE No. 3, Commercial Blast Cleaning, describes blast cleaning until at least two-thirds of the surface is free of all visible residues, with staining only permitted on the remainder.
- SSPC-SP 10/NACE No. 2, Near-White Blast Cleaning, describes blast cleaning to near-white metal cleanliness, until at least 95% of the surface is free of all visible residues, with staining only permitted on the remainder.
Inspection and Acceptance of Prepared PCP
Once the secondary surface preparation is complete, the prepared surface should be inspected before applying the final coating system. Because the prepared surface will contain intact primer, the measurement of surface profile is appropriate only on areas that have been cleaned to bare metal. Key considerations are visual cleanliness and non-visible salt contamination.
There are several visual standards for prepared PCP. This section will discuss the visual standards published by two coating manufacturers and JSTRA. These visual standards include photographs and descriptions for comparing the prepared surfaces but do not contain physical test procedures. Most manufacturers refer to washing and blowing down in addition to mechanical methods for preparing PCP. However, the visual standards predominantly address the extent and condition of intact paint remaining after mechanical surface preparation techniques. The inspection standards describe levels of surface preparation but do not provide guidance on acceptance criteria prior to overcoating (i.e., what level of cleanliness is required). Such guidance is commonly contained in the product data sheet for the main coating system.
Proprietary Abrasive Sweep Standards for Shop-Primed Steel
A major coating manufacturer has published a visual standard that provides nine photographs representing three levels of surface preparation for three different shop primers.9 The shop primers are differentiated by color— red oxide, green, and gray. For each of the shop primers, three levels of preparation are illustrated.
Standards for the Preparation of Steel Substrates for PSPC
The JSTRA published a visual standard in 2008 that contains 84 reference photographs representing a variety of conditions prior to surface treatment and after various surface treatments. The surface preparation levels include those that would comply with PSPC requirements of Sa 2½ or St 3, as well as a higher grade (designated Sa 2½+ or St 3+ in the standard).
For damaged areas, the standards provide representative surfaces cleaned using abrasive blasting (Sa 2½) or power tools (St 3). A reference condition and preparation to Sa 2½, St 3, Sa 2½+, or St 3+ is shown for damage from spot rust, light pitting, burned, manual weld, semi-automatic CO2 fillet weld, semi-automatic CO2 butt weld, automatic butt weld, collar plate weld, and corner weld.
For undamaged shop primer with surface contamination, the visual standards show cleaning to recommended conditions using abrasive blasting (Sa 2½ or Sa 2½+) for light fume by steel cutting, heavy fume by steel cutting, zinc salt, slightly heated, and fume by welding.
The visual standards show cleaning to recommended conditions using power tools (St 3, St 3+) for zinc salt, fume by welding, surface overdue for overcoating. For oil contamination, a photograph of “other cleaning” to recommended condition is shown. The other cleaning appears to be some sort of solvent wipe.
Proprietary Secondary Surface Preparation Visual Standards
A major coating manufacturer produced a visual standard for secondary surface preparation.10 The standard contains 30 photographs representing five levels of surface preparation for each of six surface types.
Past studies and the present work conclusively demonstrate that certain PCPs can be retained without impacting coating performance.
There are various PCP materials. Their compatibility with the permanent system should be confirmed by proper testing.
Secondary surface preparation requirements and acceptance criteria must be developed and agreed upon to effectively retain PCP for service.
This article is the result of work sponsored by the National Shipbuilding Research Program Surface Preparation and Coatings Committee. The author gives special thanks to Steve Cogswell (BAE Southeast Shipyards), Judie Blakey (NASSCO), and Ben Fultz (Bechtel) for their support of this project.