Repairing Bare or Damaged Areas of Galvanized Steel
The initial phases of the galvanizing process are designed to ensure clean steel surfaces, promoting the metallurgical bonding between zinc and steel and a completely galvanized fabrication. Galvanizers’ quality-assurance practices will determine if in-plant touch-up or repair is warranted in order to ensure that products leaving the galvanizing facility meet the standards of ASTM A 123/123M.
Although galvanized coatings are highly abrasion resistant, coating damage may occur once the galvanized steel leaves the galvanizer’s facility due to extremely rough handling, installation techniques, or in-service conditions. There are methods available to ensure post-galvanizing coating integrity.
ASTM A 780 authorizes three accepted touch-up and repair methods:
The touch-up and repair method chosen should consider the specific use of the galvanized steel and the performance characteristics of each of the three methods. Corrosion protection should always be the primary consideration, but certain uses and conditions may warrant selection on the basis of other performance characteristics.
According to ASTM A 780, organic or inorganic zinc-rich paint for touching up and/or repairing galvanized steel must contain between 65-69% zinc by weight, or >92% by weight metallic zinc in dry film. Inorganic zinc-rich paints are more effective than organic in terms of delivering corrosion protection, and they do not shrink after drying/curing as organic coatings do. This is a major advantage when the paint is applied to corners, edges and rough surfaces.
In keeping with 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, bare metal, for less aggressive field conditions. When abrasive-blasting or power-tool cleaning is not practical, hand-tools may be used. In all cases, surfaces must be free of oil, grease, weld flux, paint and corrosion byproducts. Abrasive blast-cleaning must extend into the surrounding undamaged galvanized coating.
Zinc-rich paints are formulated to deliver an excellent color match for both bright galvanized coatings and for matte gray galvanized coatings.
Timing of Application
Painting should take place as soon as possible after surface preparation and prior to development of visible oxides.
Zinc-rich paint is brush- or spray-applied to a clean, dry steel surface. Spraying or brushing should be in a single application of multiple passes according to the paint manufacturer’s instructions. Zinc painting can be done on the job-site and is the easiest repair method to perform because limited equipment is required.
According to ASTM A 780, the renovated area shall have a zinc coating thickness of 150% of that specified in ASTM A 123 for the thickness grade in the appropriate material category, but not more than four mils. Thickness measurements can be taken with either a magnetic, electromagnetic or eddy current gage.
- Some paints containing zinc dust provide enough friction resistance to allow them to be used on faying surfaces, delivering a coefficient-of-friction equal to or greater than the values obtained by abrasive-blasting steel surfaces.
- Inorganic coatings provide effective service at temperatures up to 700 F (370 C). Organic zinc-rich paints do not have the temperature resistance of inorganic zinc-rich paints and are limited to service temperatures of 200-300 F (90-150 C).
- If high-humidity and/or low-temperature conditions exist during painting, adhesion may be adversely affected.
- Exposure conditions will determine true corrosion protection performance.
- Care must be taken not to apply inorganic coatings too thickly, as mud-cracking may occur.
- Uniformity of the coating is largely dependent upon the skill of the worker applying the paint.
- Bond strength for paints containing zinc dust is on the order of a few hundred psi. Adhesion of the paint is largely a function of surface preparation/cleanliness.
- There is no metallurgical bond between paint and the metal surface, only mechanical.
- Abrasion resistance of zinc-containing paint is minimal compared to that of hot-dip galvanized surfaces.
- The limited ductility of zinc-rich paints gives them poor impact resistance.
- Proper curing of the repaired area must take place before the article is placed in service.
Soldering with Zinc-based Alloys
Soldering with zinc-based alloys consists of applying zinc alloy -in stick or powder form -to the area to be repaired, which has been preheated to approximately 600 F (315 C). Common repair solders include zinc-tin-lead, zinc-cadmium and zinc-tin-copper alloys.
According to ASTM A 780, the surface to be reconditioned shall be wire-brushed, lightly ground, or mild blast-cleaned. All weld flux and spatter must be removed by mechanical methods if wire brushing or light abrasive-blasting is inadequate. The cleaned area should be preheated to 600 F (315 C) and wire-brushed at the same time. Care must be taken to not burn the surrounding galvanized coating. When the repair has been completed, flux residue can be removed by water rinsing or wiping with a damp cloth.
According to ASTM A 780, the solder-renovated area shall have a zinc coating thickness at least as much as that specified in ASTM A 123 for the thickness grade in the appropriate material category, but not more than four mils. Thickness measurements can be taken with either a magnetic, electromagnetic or eddy-current gauge.
- Some cathodic and barrier protection is provided by solders.
- If solder material chemistry is chosen to match the galvanized coating, solders can deliver a very good color match.
- Because surfaces to be repaired are heated to 600 F (315 C) there may be some alloying between the base metal and the zinc. Thus, bond strength for solders is very good.
- Solders can withstand constant temperature exposure of approximately 550 F (285 C); the surrounding galvanized steel coating performs well to constant temperature exposure of about 390 F (200 C).
- Solders are the most difficult of the three repair methods to apply.
- Caution must be taken while heating the bare spot to prevent oxidizing the exposed steel or damaging the surrounding galvanized coating.
- Because solders are molten when applied, resultant coatings are inherently thin.
- Solders are typically not economically suited for touch-up of large areas because of the time involved in the process and because it is difficult to heat large areas to the same temperature.
- Operator skill is particularly important to ensure consistent coating thickness across the repair area.
- Because of the relatively thin film of zinc that can be applied, long-term performance tests of zinc-based solders indicate they do not perform as well as metalizing or zinc-rich paint.
- Based upon tests performed according to ASTM D 968, Determination of Abrasion Resistance by the Falling Sand Method, solders’ abrasion resistance is minimal compared to hot-dip galvanized—and even metalized—surfaces.
- Solder materials have little or no effect on the mechanical properties of the underlying steel.
- In general, solders have a smooth surface with very low coefficients-of-friction and should not be used in the area of faying surfaces.
- The size and location of the article to be repaired and the skill level of the repair labor are the primary determinants for economic consideration.
Metallizing is the melting of zinc powder or zinc wire in a flame or electric arc and projecting the molten zinc droplets by air or gas onto the surface to be coated. The zinc used is nominally 99.5% pure or better. The performance of wire versus powder is comparable. Zinc-aluminum alloys also can be used. The application equipment used may limit the concentration of aluminum.
Timing of Application
Zinc spraying of the clean, dry surface by skilled workers should take place as soon as possible after preparation (within four hours) and prior to development of visible oxides. Spraying in horizontal overlapping lines yields a more uniform thickness than the crosshatch technique. The zinc coating can be sealed with a thin coating of low viscosity polyurethane, epoxy-phenolic, epoxy, or vinyl resin. For details of the application sequence and procedures, consult ANSI/AWS C2.18-93.
According to ASTM A 780, the surface to be reconditioned shall be blast cleaned to SSPC-SP5/NACE No. 1, white metal, and be free of oil, grease, weld flux residue, weld spatter and corrosion products. The blast cleaning must extend into the surrounding undamaged galvanized coating. When blast cleaning is not available, wire brushing to expose bare steel is allowed.
The renovated area shall have a zinc coating thickness at least as thick as that specified in ASTM A 123/123M for the thickness grade in the appropriate material category. Thickness measurements can be taken with either a magnetic, electromagnetic or eddy current gauge.
- Abrasion resistance of zinc spray coatings is relatively moderate.
- As with hot-dip galvanizing, metalizing can improve some properties such as surface friction coefficients and corrosion fatigue resistance.
- Sprayed zinc coatings, similar to hot-dip galvanized, are suitable for constant exposures up to approximately 390 F (200 C) and short-term exposure at higher temperatures.
- Touch-up and repair by metal spraying of a surface area surrounded by galvanized steel 18 months or older delivers an excellent color match. Some sprays with aluminum additives may also be a good match for shiny galvanized surfaces.
- Zinc spraying can be done on the job site, but transporting blasting and metal spraying equipment to the job site may make it uneconomical compared to other touch-up and repair methods.
- If high-humidity conditions exist during spraying, adhesion may be degraded.
- The usual criterion for determining the expected service life of zinc coatings is thickness: the thicker the coating, the longer the service life. This is acceptable when comparing coatings produced by the same process. However, because zinc sprays have a coating density less than that of hot-dip galvanized coatings, 1.9 mils (48 microns) of zinc spray are needed to provide the same amount of zinc as 1.7 mils (43 microns) of hot-dip galvanized coating. Performance equivalency cannot be inferred from this; exposure conditions will determine true performance.
- Uniformity of the coating depends largely upon the skill of the worker.
- Adhesion of the zinc spray to the base metal is by mechanical means and depends upon the quality of surface preparation and cleaning. The higher the surface profile, the better the mechanical bond. Adhesion values of 1000 psi (7 MPa) are typical.
- The temperature of the zinc upon impact with the base metal is not high enough to result in the alloying produced by hot-dipping.