Surface treatment degreasing

FIGURE 16.2 Effect of surface treatment on the durability of epoxyaluminum joints exposed to room temperature water immersion. (1) Anodized, (2) grit-blasted plus vapor degrease, (3) vapor degrease, (4) chromate conversion coating.18... 

The general sequence of surface preparation for ferrous surfaces such as iron, steel, and stainless steel consists of the following methods degreasing, acid etch or alkaline clean, rinse, dry, chemical surface treatment, and priming. The chemical surface treatment step is not considered a standard procedure, but it is sometimes used when optimum quality joints are required. It consists of the formation of a corrosion-preventing film of controlled chemical composition and thickness. These films are a complex mixture of phosphates, fluorides, chromates, sulfates, nitrates, etc. The composition of the film may be the important factor that controls the strength of the bonded joint. 

Titanium is widely used in aerospace applications that require high strength-to-weight ratios at elevated temperatures. As a result, a number of different prebonding surface preparation processes have been developed for titanium. These generally follow the same sequence as for steel and other major industrial metal substrates degrease, acid-etch or alkaline-clean, rinse and dry, chemical surface treatment, rinse and dry, and finally prime or bond. Mechanical abrasion is generally not recommended for titanium surfaces. 

Open Flames or Hot Surfaces No degreaser should be installed near open flames or near high-temperature surfaces (above 750°F [399°C]). Welding and heat treatment operations and space heaters should not be located in proximity to solvent degreasing equipment. When these operations are in the same general area as solvent degreasing equipment, precautions should be taken, such as enclosures and local ventilation to ensure that no traces of solvent vapors enter these areas. 

This is the most important step in the surface treatment. The purpose is primarily to get a surface with high cleanliness and adequate roughness. The pre-treatment can mainly be divided in two groups a) degreasing and b) removal of mill scale and rust, and simultaneously roughening the surface. 

In terms of surface cleaning or degreasing practices, the requirements are that gross levels of extraneous material are removed. This includes friable oxides, mill scale, grease, residues of corrosion inhibitors and protective wraps. This is readily achieved by the use of the appropriate surface treatment. 

Surface treatment of Si wafers as purchased usually starts with degreasing by sonication or rinsing in organic solvents. It is a good idea to use two kinds of solvents. 

Although the exact conditions were not specified in their paper, it is reasonable to assume that the results referred to specimens prepared with solvent-degreased adherends, whereas those of Brewis et al were chromic acid etched. The differences in behavior observed in the two studies therefore probably arise because of the different surface treatments. The more durable chromic acid-etch interface was not appreciably attacked during the time of exposure, while the solvent-degreased interface was. 

Steel. Not all metal adherends require chemical surface treatments in order to optimize joint durability. With mild steel, removal of soluble contaminants by vapor degreasing followed by grinding or grit blasting is sufficient (91). However, the freshly created surface of steel is very reactive and reoxidizes almost instantly. It will continue to oxidize, especially in the presence of moisture, eventually forming a visible rust. The treated surface must be coated with a primer or adhesive before the oxide layer becomes too thick, otherwise joint strength and durability will be poor (92). 

Abrasive Developments have, in conjunction with their Japanese licensee, developed a wet blast phosphating plant that raises the quality standard within the industry. The solution achieved delivers high quality components from an automatic machine that combines both the cleaning and phosphating processes. The cleaning section benefits from the unique degreasing and surface treatment properties of the VAQUA process. 

Many operations involve the use of solvent tanks for surface treatment, metal cleaning, degreasing, stripping and acid treatment. Canopy receiving hoods are sometimes utilized to collect contaminants from some of these operations however, they are most effective when used for hot processes. The engineer must consider air movement and currents which may allow the release of the contaminant into the workplace when designing such a system. In addition, some processes may involve the movement of workers heads over the tank or the lifting of... 

Fig. 2.17. Strengths of some single lap joints of aluminium bonded with an epoxy-polyamide adhesive on exposure to air at 43°C and 90% relative humidity for various times. Metal surface treatments are etching in chromic-sulphuric acid, O alkaline etching, solvent degrease, and phosphoric acid etching, (after Butt and Cotter ).

Aluminum and aluminum alloys. The effects of various aluminum surface treatments have been studied extensively. The most widely used process for high-strength, environment-resistant adhesive joints is the sodium dichromate-sulfuric acid etch, developed by Forest Product Laboratories and known as the FPL etch process. Abrasion or solvent degreasing treatments result in lower bond strengths, but these simpler processes are more easily placed into production. Table 7.14 quahtatively lists the bond strengths that can be realized with various aluminum treatments. 

Numerous studies are underway to find surface treatment methods that minimize or eliminate use of toxic materials or polluting substances. Some progress is being made, " but replacement of such degreasing solvents as trichloroethane and such toxic materials as the dichromates will take time. For polymeric surfaces, plasma or corona discharge treatments are receiving much attention. 

Aluminium pieces assembled with fittings made in ordinary steel, stainless steel or copper are commonly degreased in trichloroethylene or perchloroethylene vapours in surface treatment shops. When performed in a stabilised solvent (containing additives that... 

The technological implications of the above points are basically that, to attain good initial joint strengths, in many instances no surface treatment is necessary and even when the state of the incoming substrate material is such that relatively thick layers of protective greases, mill scale, etc. need to be removed then a simple degreasing and, possibly, mechanical abrasion, pretreatment are all that is required. 

Abstract The use of surface treatments to optimize adhesion has been well established. In this chapter, the main treatment methods for metals and polymers will be considered in terms of how such processes are carried out and their influence on surface physical and chemical properties. Consideration has been given to a range of treatments from simple degrease options to the more highly complex multistage processes. An attempt has been made to relate the changes in physicochemical properties to adhesion performance, and, where relevant durability. 

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