Surface Treatments for Metals

The ability to clean and prime metals with environmentally-compliant and environmentally-friendly chemicals is being studied in all industries to maximise adhesive strength and enhance long-term durability. 

Corrosion has been the major factor causing joint failure in the aerospace industry and this is still an issue facing potential industrial and automotive users, particularly if metals such as steel or copper are used [20]. Researchers based in Japan [21] showed that acid treatment of steel does not improve the fracture toughness of bonds with toughened epoxies, but it does increase the fatigue growth 

In the aerospace industry, aluminium and alloys are used widely and there are essentially two steps used to prepare these surfaces for bonding, i.e., an etching or anodisation process (to maximise adhesive strength) and a priming process (to prevent corrosion of assembled joints). 

For aluminium alloys, there are three common surface-preparation 

Primers are also used and are typically organic solvent-based solutions of epoxies or phenolic adhesives. They provide several major functions  

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The Use of Electrochemical Techniques in the Study of Surface Treatments for Metals and Alloys... 

Chapter 4 describes how the electrical nature of corrosion reactions allows the interface to be modeled as an electrical circuit, as well as how this electrical circuit can be used to obtain information on corrosion rates. Chapter 5 focuses on how to characterize flow and how to include its effects in the test procedure. Chapter 6 describes the origins of the observed distributions in space and time of the reaction rate. Chapter 7 describes the applications of electrochemical measurements to predictive corrosion models, emphasizing their use in the long-term prediction of corrosion behavior of metallic packages for high-level nuclear waste. Chapter 8 outlines the electrochemical methods that have been applied to develop and test the effectiveness of surface treatments for metals and alloys. The final chapter gives experimental procedures that can be used to illustrate the principles described. 

An example of ultrasonic treatment of metals is in electrolytic polishing. This is an economical method of surface treatment for metal workpieces and in particular delicate products such as metal filters made from fine wire. These products can be finished more rapidly and economically by this method than by usual mechanical cleaning techniques. The condition of the electrolyte in this case plays an important part. Investigations on ultrasonically treated electrolyte solutions have shown that they enable workpieces to be polished in half the time required by conventional solutions. In addition it is possible to attain improved metal structures with these electrolytes as a result of their particularly aggressive action on the surface of the product [112]. 

Thus the theory has proved a useful one in the sense that it has stimulated the development of new surface treatments for metals, polymers and fibers and has assisted in giving an understanding of their efficacy. There has perhaps been a tendency, now that the theory is again respectable, to invoke mechanical effects somewhat uncritically wherever an increase in surface roughness has been observed. A more detailed review of these developments may be found in references [18] and [44]. 

Secondly, to move away from the very complex multi-stage surface pretreatments which are typically needed to give the most durable joints, but are expensive and present health and safety problems. Hence, there is a need to develop surface treatments for metallic substrates, such as the organosilane primers, which will... 

The benefits of atmospheric plasma surface treatment for metal surfaces have been well documented. Most metal surfaces are rendered highly hydrophilic and wettable with gas-phase approaches. Surface bonding of metal coatings is enhanced, as are polymer depositions. Surface sterilization and removal of microbial contaminations and biofilms from metal-based medical devices is common for biomedical applications and for metal blomaterials. Complex contoured metal geometries can be readily processed by three-dimensional plasma discharge devices. 

This conversion is normally accompHshed by immersion, but spraying, swabbing, bmshing, and electrolytic methods are also employed (178) (see Metal SURFACE treatments). The metals that benefit from chromium surface conversion are aluminum, cadmium, copper, magnesium, silver, and 2inc. Zinc is the largest consumer of chromium conversion baths, and more formulations are developed for 2inc than for any other metal. 

Although many standards exist for cleaning treatments for metal surfaces, for example Defence Standard DEF STAN 03-2/1, these are often fairly general guides which in some cases may be regarded as somewhat outdated due to recent advances in treatment technology and changes in industrial practice. 

Primers are pigmented coatings that are applied to new surfaces or to old cleaned surfaces, prior to the application of undercoats or top coats. Its main functions are to achieve adequate adhesion to the substrate and to provide good intercoat adhesion for subsequent coats. They are specifically formulated for particular substrates such as wood, metals, concrete, and other masonry surfaces. Concrete and other masonry surfaces are alkaline and often require special surface treatments. For etching and neutralization of these alkaline surfaces, hydrochloric or phosphoric acid washing is usually done. 

The working electrode is a key factor in the process, directing the course of the electrochemical reaction according to its properties material, adsorbent surface, etc. The working electrode must be stable towards corrosion and may be improved by additives or surface treatments. For bulk electrolysis, a high surface to volume ratio is chosen in order to reduce the electrolysis time. The electrodes may be constructed of grids, foams, expanded metals, liquid mercury, porous material, wool, etc. For electrocrystallization experiments, the size is less important and generally platinum wires are used. 

Fig. 5.63 a Electrically heated oven for the surface treatment of metal plates, b Hollow enclosure for the calculation of the radiative exchange of the top half of the oven... 

In addition to the characterization of the lamellar arrangement in processed polymers, as discussed above, AFM is useful to analyze changes in surface morphology and roughness owing to surface treatments. For example, Vancso and coworkers reported on an early AFM study of the surface morphology of commercial polymer films. The effects of various oxidizing treatments and of metallization of PP have been unraveled. In this study the thickness and orientation of the exposed lamellae were determined [161]. 

Beilstein Handbook Reference) AI3-62190 Benzene, 1,r-(sulfinylbis(methylene))bis- Benzyl sulfoxide BRN 2049262 Dibenzyl sulfoxide Dibenzyl sulphoxide EINECS 210-668-7 NSC 55 Preventol Cl 5 Sulfoxide, dibenzyl Tardiol D. An organic inhibitor for use in cleansing acids and in the surface treatment of metals. Leaflets mp = 134° bpn 210° (dec) Xm = 220, 253, 260, 266, 270 nm (MeOH) insoluble in H2O, soluble in EtzO,... 

P. A. M. Basheer, Surface Treatments for Concrete , COST 509 Workshop Corrosion and Protection of Metal in Contact mth Concrete, Sevilla, 4-9 September 1995. 

Teflon and other fluorocarbon surfaces may be altered by chemical treatments using solvent solutions of sodium naphthalene complex or molten sodium. Teflon etchants, for example, Tetra-Etch , a tradename and product of W. L. Gore Associates, are effective in improving adhesion to Teflon. Oxidation treatments for metals such as copper or aluminum also produce highly polar oxide surfaces that are more easily bondable. The reader is referred to several sources for a more comprehensive... 

F.N. Rhines. Surface Treatment of Metals, 122, American Society for Metals, Cleveland, OH (1941). 

Reference Document on Best Available Techniques for the Surface Treatment of Metals STM... 

Conversion coatings. Some metals can be treated by oxidizing agents, preferentially chromate solutions, to prepare an oxide film saturated by chromate ions. This is one of the most effective corrosion preventing treatments of metal surfaces and is applied to aluminum, copper, and zinc. Chromate ions are known for toxic properties. Pohtical decisions have banned this surface treatment for the future. So far no alternative corrosion protecting treatment of similar effectiveness has been developed. 

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