Metal Glue tips: How Do I Make Paint Or Adhesives Stick to Metals?
At least half the technology of adhesive bonding or coating application is surface preparation. Here we share the metal glue tips and techniques that show you how to prepare surfaces to form a strong adhesive bond for a range of common metals.
Copper and copper alloys (brass and bronze) are chemically different from iron, zinc-plated (galvanized) iron, nickel-iron alloys (stainless steel), aluminium alloys, or lead, and all of these are different from each other. Each of these requires a different surface preparation procedure with different chemicals in order to obtain a good adhesive bond.
A separate application note deals with the detailed steps of coating systems for steel boat hulls, architectural ironwork and similar items. Metal glue tips is intended to give an overview of the chemical requirements for all metals.
Lead Surface Preparation
In the case of lead, the best that can be done is to sand the lead to a bright surface with 36 grit and then apply our High-Build. epoxy paint within a day or so. The bond that will be achieved is not ideal compared to what can be obtained from other metals, but it is the best that can be done at the present state of the art (1999).
Iron And Galvanised (Zinc Plated) Iron Surface Preparation
Both iron and zinc-plated iron may be prepared for painting or bonding by treating with phosphoric acid or one of a wide variety of products containing phosphoric acid (such as mordant solution or T-Wash). The phosphoric acid reacts chemically with the metal to product a metal phosphate. Physically, this film is a dense mat of crystals which are chemically bonded to the base metal they grow out of. The paint or glue sticks to the mat of phosphate crystals as well as paint sticks to a rug. All phosphate products change the colour of the underlying metal.
Copper And Copper Alloy Surface Preparation
Copper and its alloys may be prepared for bonding or coating by applying a black oxide finish. This is a complicated process that must be done by a plating or metal finishing company using special procedures. It is not suitable for use in the field. Where this procedure is not appropriate, there is a second option which is almost as good but requires excellent workmanship. Good bonds to copper may be obtained if the surface is chemically clean. Unfortunately, within half an hour a chemically clean copper or copper alloy (brass, bronze, cupronickel) surface will grow a copper oxide film by reaction with the oxygen in the air. The oxide film is physically weak and does not stick well to the underlying metal, and so only poor bonds will be obtained. The chemically clean surface may be obtained by an application of our Copper/Brass/Bronze prep solution, a prompt rinse with distilled or deionized water, and a prompt drying with clean paper towels or a rinse with pure Isopropyl Alcohol. The rinsing and drying materials must be free of sulphur, which (similarly to oxygen) would react with the copper and not allow for a good bond. Tap water often contains traces of sulphur, and the air around heavily industrialized or populated areas contains hydrogen sulphide and oxides of sulphur. Therefore, use clean materials and work quickly. The time from rinsing through drying to bonding should not exceed one hour at 72øF (20øC) and half that time at 90øF (30øC). Epoxies bond readily to such a surface. Polysulphide adhesives should not be used.
Aluminium Surface Preparation
For aluminium, which corrodes (oxidises) in air readily (it grows a physically weak oxide film in a few hours), there are three options.
The same oxide film, if grown electrically (by passing an electric current through an electric cell containing aluminium and a solution of sulphuric acid), is clear or coloured, hard, and protects the aluminium against corrosion. This process is called Anodizing and is not suitable for use in the field. Anodized aluminium is commercially available and epoxy or polysulphide adhesives and most single component caulks should bond well to anodized aluminium. However, some anodized aluminium is treated with a wax or something similar after the anodizing process, and adhesives may not stick. Always do a small test first.
For best results in the field, the clean, sanded or chemically etched aluminium surface should be treated with a product called Alodine. That product grows a chromate complex on the surface, and paints or adhesives develop good bonds. The chromate complex offers a degree of inhibition of corrosion and is a golden or tan colour. Smith & Co. offers a dry concentrate of the Alodine chemical which may be added to 1/2 – 1 gallon water to make the Alodine solution.
Stainless Steel Surface Preparation
For stainless steel and high-nickel alloys such as Monel, special acids are necessary which are dangerous to handle in their concentrated form and the unused quantities are dangerous to store and expensive to dispose of. Smith & Co. at present does not have surface preparation products for such metals, but can produce them on special order.