This article has multiple issues. Unsourced material may be challenged and removed. The purpose of applying the coating may be decorative, functional, or both. The coating itself may be an all-over coating, completely covering the substrate, or it may only cover parts of metal coating process pdf substrate.
A major consideration for most coating processes is that the coating is to be applied at a controlled thickness, and a number of different processes are in use to achieve this control, ranging from a simple brush for painting a wall, to some very expensive machinery applying coatings in the electronics industry. A further consideration for ‘non-all-over’ coatings is that control is needed as to where the coating is to be applied. Decorative- often to impart a specific colour, but also to create a particular reflective property such as gloss or matt. Slide coating- bead coating with an angled slide between the slotdie and the bead. Very successfully used for multilayer coating in the photographic industry. Slot die bead coating- typically with the web backed by a roller and a very small gap between slotdie and web. Tensioned-web slotdie coating- with no backing for the web.
Blunt, Engineering Coatings: Design and Application, Woodhead Publishing Ltd, UK, 2nd ed. Epoxy Coatings for Automotive Corrosion Protection”. Coating Materials for Electronic Applications: Polymers, Processes, Reliability, Testing by James J. This page was last edited on 11 December 2017, at 15:10.
This oxide remains conformal even when plated on wire and the wire is bent. Anodized aluminium surfaces, for example, are harder than aluminium but have low to moderate wear resistance that can be improved with increasing thickness or by applying suitable sealing substances. Anodic films are generally much stronger and more adherent than most types of paint and metal plating, but also more brittle. This makes them less likely to crack and peel from aging and wear, but more susceptible to cracking from thermal stress. It is still used today despite its legacy requirements for a complicated voltage cycle now known to be unnecessary. Variations of this process soon evolved, and the first sulfuric acid anodizing process was patented by Gower and O’Brien in 1927.
Sulfuric acid soon became and remains the most common anodizing electrolyte. Oxalic acid anodizing was first patented in Japan in 1923 and later widely used in Germany, particularly for architectural applications. The phosphoric acid processes are the most recent major development, so far only used as pretreatments for adhesives or organic paints. A wide variety of proprietary and increasingly complex variations of all these anodizing processes continue to be developed by industry, so the growing trend in military and industrial standards is to classify by coating properties rather than by process chemistry. However, anodizing does not increase the strength of the aluminium object. 15 nm thick, but tend to be more susceptible to corrosion. Aluminum alloy parts are anodized to greatly increase the thickness of this layer for corrosion resistance.
Although anodizing produces a very regular and uniform coating, microscopic fissures in the coating can lead to corrosion. To combat this, various techniques have been developed either to reduce the number of fissures, to insert more chemically stable compounds into the oxide, or both. For instance, sulfuric-anodized articles are normally sealed, either through hydro-thermal sealing or precipitating sealing, to reduce porosity and interstitial pathways that allow corrosive ion exchange between the surface and the substrate. Precipitating seals enhance chemical stability but are less effective in eliminating ion exchange pathways. Most recently, new techniques to partially convert the amorphous oxide coating into more stable micro-crystalline compounds have been developed that have shown significant improvement based on shorter bond lengths. Some aluminium aircraft parts, architectural materials, and consumer products are anodized.
Although anodizing only has moderate wear resistance, the deeper pores can better retain a lubricating film than a smooth surface would. Anodized coatings have a much lower thermal conductivity and coefficient of linear expansion than aluminium. The coating can crack, but it will not peel. This and the non-conductivity of aluminum oxide can make welding more difficult. In typical commercial aluminium anodizing processes, the aluminium oxide is grown down into the surface and out from the surface by equal amounts. So anodizing will increase the part dimensions on each surface by half the oxide thickness. If the part is anodized on all sides, then all linear dimensions will increase by the oxide thickness.
Anodized aluminium surfaces are harder than aluminium but have low to moderate wear resistance, although this can be improved with thickness and sealing. The voltage required by various solutions may range from 1 to 300 V DC, although most fall in the range of 15 to 21 V. Higher voltages are typically required for thicker coatings formed in sulfuric and organic acid. However, these same pores will later permit air or water to reach the substrate and initiate corrosion if not sealed. Because the dye is only superficial, the underlying oxide may continue to provide corrosion protection even if minor wear and scratches may break through the dyed layer. Conditions such as electrolyte concentration, acidity, solution temperature, and current must be controlled to allow the formation of a consistent oxide layer. Harder, thicker films tend to be produced by more dilute solutions at lower temperatures with higher voltages and currents.
The film thickness can range from under 0. 150 micrometers for architectural applications. Each process provides corrosion resistance, with anodizing offering a significant advantage when it comes to ruggedness or physical wear resistance. The reason for combining the processes can vary, however, the significant difference between anodizing and chromate conversion coating is the electrical conductivity of the films produced. Although both stable compounds, chromate conversion coating has a greatly increased electrical conductivity. Applications where this may be useful are varied, however, the issue of grounding components as part of a larger system is an obvious one. The dual finishing process uses the best each process has to offer, anodizing with its hard wear resistance and chromate conversion coating with its electrical conductivity.