Practice for Repair of Damaged and Uncoated Area of Hot Dip Galvanize Coating (According to A 780)

If the galvanized product does not meet all of the requirements of the specification, it must be repaired or rejected along with the lot it represents. When repair of the product is allowed by the specification or bare spots are present, the galvanizer is responsible for the repair unless directed otherwise by the purchaser. The specifications allow for some retesting of products that represent lots or retesting after the lot has been sorted for non-conformance. The coating thickness of the repaired area must match the coating thickness of the surrounding area. The maximum sizes for allowable areas that can be repaired during in-plant production are defined in ASTM A 123/A 123 M. The method for repair of damaged and uncoated area of hot dip galvanizes coating according to A 780 as below:

1.0 Zinc-Based Solder

Any repairs made to galvanized products must follow the requirements of ASTM A 780, which defines the acceptable materials and the required procedures. Repairs are normally completed by the galvanizer before the products are delivered, but under certain circumstances, the purchaser may perform the repairs on their own. The touch-up and repair materials are formulated to deliver an excellent color that matches either brightly coated, newly galvanized products or matte gray, aged galvanized products. Materials used to repair hot-dip galvanized products include zinc-based solder, zinc-rich paint, and zinc spray metalizing, and are explained in the following sections.

Soldering with zinc-based alloys is achieved by applying zinc alloy in either a stick or powder form. The area being repaired needs to be preheated to approximately 600 F (315 C). The most commonly used solders for repair; include zinc-tin-lead, zinc-cadmium, and zinc-tin-copper alloys.

1.1 Surface Preparation

According to ASTM A 780, the surface to be reconditioned shall be wire brushed, lightly ground, or mildly blast cleaned. In addition, if wire brushing or light blasting is inadequate, all weld flux and spatter must be removed by mechanical methods. The cleaned area also needs be preheated to 600 F (315 C) and wire brushed while heated. Pre-flux may also be necessary to provide chemical cleaning of the bare spot. Finally, special care should be given to insure that the surrounding galvanized coating is not overheated and burned by the preheating.

1.2 Application

The soldering method is the most difficult of the three repair methods to complete. A high level of caution must be taken while heating the bare spot to prevent oxidizing the exposed steel or damaging the surrounding galvanized coating. Solders are typically not economically suited for touch-up of large areas because of the time involved in the process and because heating of a large surface area to the same temperature is very difficult. When the repair has been completed, the flux residue needs to be removed by rinsing the surface with water or wiping with a damp cloth.

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1.3 Final Repaired Product

The final coating thickness for this repair shall be agreed upon between the galvanizer and the purchaser, and is generally in the 1 to 2 mil range. The thickness shall be measured by any of the methods in ASTM A 123/A 123M that are non-destructive. Zinc-based solder products closely match the surrounding zinc and blend in well with the existing coating appearance.

2.0 Zinc Rich Paint

Zinc-rich paint is applied to a clean, dry steel surface by either a brush or spray and usually contains an organic binder pre-mix. Zinc-rich paints must contain either between 65% to 69% metallic zinc by weight or greater than 92% metallic zinc by weight in dry film. Paints containing zinc dust are classified as organic or inorganic, depending on the binder they contain. Inorganic binders are particularly suitable for paints applied in touch-up applications around and over undamaged hot-dip galvanized areas.

2.1 Surface Preparation

According to ASTM A 780, the surface to be repaired shall be blast cleaned to SSPC-SP10/NACE No.2 near white metal for immersion applications and SSPC-SP11 near bare metal for less aggressive field conditions. When blasting or power tool cleaning is not practical, hand tools may be used to clean areas to be reconditioned. The blast cleaning must extend into the surrounding, undamaged, galvanized coating.

2.2 Application

This method of repairing galvanized surfaces must take place as soon as possible after preparation is completed and prior to the development of any visible oxides. The spraying or brushing should be in an application of multiple passes and must follow the paint manufacturer’s specific written instructions. In addition, proper curing of the repaired area must occur before the product is put through the final inspection process. This repair can be done either in the galvanizing plant or on the job site and is the easiest repair method to apply because limited equipment is required. Zinc-rich painting should be avoided if high humidity and/or low temperature conditions exist because adhesion may be adversely affected.

2.3 Final Repaired Product

The coating thickness for the paint must be 50% higher than the surrounding coating thickness, but not greater than 4.0 mils, and measurements should be taken with either a magnetic, electromagnetic or eddy current gauge. Finally, the surface of the painted coating on the repaired area should be free of lumps, coarse areas, and loose particles.

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3.0 Zinc Spray Metalizing

Zinc spray, which is also referred to as metalizing, is done by melting zinc powder or zinc wire in a flame or electric arc and projecting the liquid zinc droplets by air or gas onto the surface to be coated. The zinc used is nominally 99.5% pure or better and the corrosion resistance of the wire or powder is approximately equal.

3.1 Surface Preparation

According to ASTM A 780, the surface to be reconditioned shall be blast cleaned to SSPC-SP5/NACE No.1 near white metal and must be free of oil, grease, weld flux residue, weld spatter and corrosion products. The blast cleaning must extend into the surrounding, undamaged, galvanized coating.

3.2 Application

Zinc spraying of the clean, dry surface must be completed by skilled workers and should take place within four hours after preparation or prior to development of visible oxides. Spraying should also be done in horizontal overlapping lines, which yield a uniform thickness more consistent than the crosshatch technique. The zinc coating can be sealed with a thin coating of low viscosity polyurethane, epoxy-phenolic, epoxy, or vinyl resin. The details of the application sequence and procedures can be found in ANSI/AWS C2.18-93. The application of zinc spray can be done either in the galvanizer’s plant or at the job site. In addition, if high humidity conditions exist during spraying, adhesion may be degraded.

3.3 Final Repaired Product

The renovated area shall have a zinc coating thickness at least as thick as that specified in ASTM A 123/A 123M for the thickness grade required for the appropriate material category. These thickness measurements should be taken with either a magnetic or an electromagnetic gauge for best results. The plain zinc sprays or the sprays with aluminum additives both provide a good match for newly galvanized, bright surfaces. Finally, the surface of the sprayed zinc coating should be free of any lumps, coarse areas, and loose particles.

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