Surface Preparation V: Metals
In plastics bonding, surface preparation is used to increase the surface polarity and improve wetting ability for the adhesive or primer. Metal surfaces typically have oxide layers. These oxide layers are already highly polar and inherently lend themselves to good adhesive bonding. Surface preparation is used to improve bonding to metal surfaces, but the main reason for preparing a metal surface is to improve the durability of the bond, particularly in moist environments.
Because metals have oxide surfaces, they attract moisture. Active metals such as aluminum, copper, nonstainless ferrous metals, magnesium and titanium undergo changes in surface chemistry when moisture interacts with surface oxides. Surface hydroxides are produced. These often have very low strength against the parent metal. Even though the adhesive bonds well to the surface, moisture attack changes the surface underneath the adhesive, and the bond fails. The purpose of surface preparation for many metals is to stabilize the oxide layer against chemical changes over time. More on that later.
Bonding EnhancementFirst, the surface should be clean. Many acrylic adhesives will tolerate some surface oil, and other adhesive types may be formulated to absorb oil, but in general, metals should be degreased prior to bonding. Solvent degreasing was common previously, but there are now many waterborne degreasers that work well at cleaning surfaces.
Metals can be grit-blasted after cleaning, which greatly improves bond strength in many cases. This is a mechanical effect by virtue of the increased surface area and the depth of surface being bonded. Other than the initial increase in bond strength, the durability of the bond is not improved by grit-blasting, meaning moisture attack will still weaken the bond. However, if the initial bond is stronger, the weakened bond will also be stronger than it would otherwise be.
There are also metals that do not bond well even though they have high polarity. These are the passivated metals such as stainless steels, nickel and its alloys, and the noble metals. If a metal is highly resistant to corrosive attack, it is often difficult to bond for the same reason. The metal resists interaction with foreign substances, and that includes adhesives. Most of those metals require extensive oxidative treatment to bond well. This may be supplied by acid oxidation or by plasma, just as with plastics. Even though the metals have oxide surfaces, the type of oxide does not interact with adhesive so the oxide layer must be modified.
Along the same lines, very smooth surfaces do not bond well. Either grit-blasting or oxidation can be useful here. The bonding increase with oxidation is attributed to the surface roughening that often accompanies oxidative attack.
Durability EnhancementTo improve the durability of the surface, it is necessary to effect a chemical stabilization. Bond durability cannot be separated from bonding enhancement because the lifetime of the bond is dependent on both. Durability refers to the retention of bonding properties over time and exposure. Usually that is an issue with the active metals as opposed to the passivated metals. For example, it has been shown that most methods for increasing bonding to stainless steels are equally effective, while those used for bonding aluminum or titanium seem to have a clear dependence on the increase in surface hydrophobicity. Grown oxide layers that are insoluble in water demonstrably increase the service lifetime for aluminum or titanium systems.
The most common method for increasing durability on active metals is anodization. Anodization is a chemical process for growing specific types of oxides into a surface using electrolytic attack in a chemical bath. Common methods are chromic acid, sulfuric acid and phosphoric acid anodizations, usually run at specified pH and applied voltage of 10-40V. Anodization is mostly used on aluminum and titanium, and usually for aerospace-bonding applications. The oxide layers are grown into the surface, producing both the specific oxide chemistry for water resistance and a surface profile for good mechanical anchoring of the adhesive or primer.
There are surface treatments that are grown out of the surface in the form of hydrophobic crystallites. These provide a good anchor surface as well as improve durability. Phosphatization of mild and galvanized steels is almost universal in the automotive industry for providing better adhesion of electrocoated primers for paint. The iron and zinc phosphate crystals produced on the surface are highly insoluble in water. This preparation can be used with adhesive bonding, as well, to improve durability.
Primers are used with either anodization or phosphatization to provide an interfacial bond between the metal oxide and the adhesive. The primers can also take the form of hygroscopic materials. A new class of silane primers has shown great promise for improving bond durability, even without anodization. These materials bond well to cleaned metal and provide very impressive water durability. Primers are an important part of the most demanding bonding operations and are often used in combination with other preparative methods. Priming protects a cleaned surface from further contamination while providing a cooperative surface-chemistry modification.
What Can I Get Away With?Because metals bond well, for the most part, the real issues with metals bonding are absolute bond strength and long-term stability. As with plastics, a lot of time can go into surface preparation to improve absolute strength. But, unlike plastics, metal surfaces can degrade underneath the adhesive with time. Because of the durability issue, bonding with metals usually requires more surface preparation than that required just for producing a good initial bond. And that usually means more money spent on the product.
The situation is further complicated by the difficulty of extrapolating test results to predict a product lifetime. The best data for correlating bond durability with short-term test results come from the aerospace industry. Those data are taken from aluminum bonding, with some relating to titanium. Predicting the life of a component with no field history is difficult to impossible. That means the conservative choice is to over-prep the surface. Critical applications require an escalating operation of: cleaning (bare-minimum requirement), grit-blasting or anodizing/phosphatizing/oxidizing, priming and then bonding. Less critical applications might do well with only a cleaning and a grit-blast.
The mental dividing line for needed preparative methods depends on how we define “high reliability,” or these days, it’s more like “high liability.” A structural bond in an application where failure could endanger people requires the best bonding techniques. Applications for use in high-humidity environments require attention to more advanced methods. Bonding for noncritical use, where a minimum bond strength only needs to be met and kept, usually does not require more than cleaning, surface roughening and good adhesive selection.
Surface preparation for bonding of metals follows the same general principles as that for plastics. Provide a clean surface with good wettability and protect the surface from deterioration with time. Prime if the surface chemistry requires modification, and use the correct adhesive for the temperature or environmental conditions expected.
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