Coatings erosion

Some tablets that provide a sustained period (up to 8—12 h) of therapy may be coated during processing. A portion is released first to bring the dmg to the desired blood concentration (onset of activity), whereas a sustained-release portion maintains an effective level for a prolonged period of time (duration of activity), eg, by coating erosion or diffusion of dmg through it. 

Intuitively, it seems likely that a decrease in crosslink density on exposure would promote coating erosion. In clearcoats, on the other hand, the most serious appearance problem is crack formation. 

High temperature protection Wear-resistant coatings Antigaiiing coatings Erosion/abrasion resistance properties [20]... 

Studies of photooxidation under constant and controlled conditions have shown that a lag time is evident prior to the onset of coating erosion. This lag time is independent of the relative humidity of the surrounding air (e.g., Rezig et al. [6]). The stable (constant) erosion rate, established shortly after the lag time is over, however, is indeed a function of the relative humidity. Rezig et al. [6] have shown that the stable erosion rate is reduced by more than 50 % by changing the RH from 75 to 9 %. 

Thermal spray processes can be used to give coatings of chromium carbide or nickel chromium for erosion resistance, copper nickel indium for fretting resistance, tungsten carbide cobalt for wear and abrasion resistance, and even aluminum siHcon polyester mixtures for abradabiHty. 

The expander turbine is designed to minimize the erosive effect of the catalyst particles stiU remaining in the flue gas. The design ensures a uniform distribution of the catalyst particles around the 360° aimulus of the flow path, optimizes the gas flow through both the stationary and rotary blades, and uses modem plasma and flame-spray coatings of the rotor and starter blades for further erosion protection (67). 

Impact and Erosion. Impact involves the rapid appHcation of a substantial load to a relatively small area. Most of the kinetic energy from the impacting object is transformed into strain energy for crack propagation. Impact can produce immediate failure if there is complete penetration of the impacted body or if the impact induces a macrostress in the piece, causing it to deflect and then crack catastrophically. Failure can also occur if erosion reduces the cross section and load-bearing capacity of the component, causes a loss of dimensional tolerance, or causes the loss of a protective coating. Detailed information on impact and erosion is available (49). 

Metal surfaces in a well-designed, well-operated cooling water system will establish an equilibrium with the environment by forming a coating of protective corrosion product. This covering effectively isolates the metal from the environment, thereby stifling additional corrosion. Any mechanical, chemical, or chemical and mechanical condition that affects the ability of the metal to form and maintain this protective coating can lead to metal deterioration. Erosion-corrosion is a classic example of a chemical and mechanical condition of this type. A typical sequence of events is ... 

Metals that depend on a relatively thick protective coating of corrosion product for corrosion resistance are frequently subject to erosion-corrosion. This is due to the poor adherence of these coatings relative to the thin films formed by the classical passive metals, such as stainless steel and titanium. Both stainless steel and titanium are relatively immune to erosion-corrosion in most cooling water environments. 

The resistance of a metal to erosion-corrosion is based principally on the tenacity of the coating of corrosion products it forms in the environment to which it is exposed. Zinc (brasses), aluminum (aluminum brass), and nickel (cupronickel) alloyed with copper increase the coating s tenacity. An addition of V2 to 1)4% iron to cupronickel can greatly increase its erosion-corrosion resistance for the same reason. Similarly, chromium added to iron-base alloys and molybdenum added to austenitic stainless steels will increase resistance to erosion-corrosion. 

But they all oxidise very rapidly indeed (see Table 21.2), and are utterly useless without coatings. The problem with coated refractory metals is, that if a break occurs in the coating (e.g. by thermal fatigue, or erosion by dust particles, etc.), catastrophic oxidation of the underlying metal will take place, leading to rapid failure. The unsafeness of this situation is a major problem that has to be solved before we can use these on-other-counts potentially excellent materials. 

Plate anodes were used for corrosion protection in order to avoid damage due to erosion and cavitation. These consisted of enamelled steel bodies in which a metal oxide-coated titanium anode 1 dm in surface area was fitted. The enamel... 

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