Corrosion—erosion characteristics

Corrosion-erosion characteristics of material removal in metal CMP... 

Thorium oxide. The major portion of studies on corrosion-erosion characteristics of high-temperature aqueous slurries has been carried out using thorium oxide [110-111]. Most preparations have used the calcination product of thorium oxalate precipitated under various conditions, with calcination temperatures from 450 to 1600°C having been used. Thoria prepared by other procedures, such as formate precipitation, has also beeti examined. Thoria calcined at temperatures in the vicinity of 1600°C has low surface area, crystallite sizes approaching particle size, less tendency to degrade, and exhibits a greater tendency to produce abrasive sintered particles consequently, certain of such thoria products have been... 

Water velocity and turbulence can damage protective films and deposits on metal surfaces causing increased corrosion. Soft metals, such as copper, are particularly susceptible to erosion corrosion, but steel and other metals are also susceptible if the water velocity is sufficiently high. The critical velocity for erosion-corrosion of copper in freshwater is about 5 s (1.52 m/s), but this velocity can drop sharply as the chemical corrosivity of the water increases. Suspended solids in the water can increase the erosion characteristics of the water [1]. Deposits can result in accelerated corrosion from the formation of oxygen differential concentration corrosion cells. 

The fluid temperature is another fundamental requirement. The maximum and minimum values as well as the history of temperature variation need to be known. The material selection is dictated to some extent by this requirement. Further requirements might involve environmental characteristics such as corrosion, erosion, and irradiation. 

For the practical utilization of circulating aqueous slurries, a suitable balance must be made between corrosion-erosion attack of materials of construction and handling properties of the slurry, including viscosity, heat transfer, settling, resuspension, and caking characteristics. The materials of construction, operating conditions, slurry characteristics, and... 

Effect of concentration. In general, corrosion-erosion by slurries has been observed to increase with concentration [115-116], and roughly is indicated to be directly proportional to concentration, in concentration ranges of reactor interest. As concentration is increased, the effect of rheological properties on flow characteristics becomes more pronounced, and the effect on corrosion would become altered. 

Corrosion/erosion resistance is one of the most important characteristics of refractories, which are exposed to molten metal and slag. Hence, the design of the tests closely simulating the conditions that the refractories experience during use is of great importance. 

Erosion-corrosion is a fairly complex failure mode influenced by both environmental factors and metal characteristics. Perhaps the most important environmental factor is velocity. A threshold velocity is often observed below which metal loss is negligible and above which metal loss increases as velocity increases. The threshold velocity varies with metal and environment combinations and other factors. 

Although it is entirely possible for erosion-corrosion to occur in the absence of entrained particulate, it is common to find erosion-corrosion accelerated by a dilute dispersion of fine particulate matter (sand, silt, gas bubbles) entrained in the fluid. The character of the particulate, and even the fluid itself, substantially influences the effect. Eight major characteristics are influential particle shape, particle size, particle density, particle hardness, particle size distribution, angle of impact, impact velocity, and fluid viscosity. 

In addition to fluid velocity, other characteristics of the eroding fluid can exert a marked influence on the erosion-corrosion process. Among the important factors are the following ... 

VFO works well in gas turbines. In a nine-month test program, the combustion properties of VFO were studied in a combustion test module. A gas turbine was also operated on VFO. The tests were conducted to study the combustion characteristics of VFO, the erosive and corrosive effects of VFO, and the operation of a gas turbine on VFO. The combustion tests were conducted on a combustion test module built from a GE Frame 5 combustion can and liner. The gas turbine tests were conducted on a Ford model 707 industrial gas turbine. Both the combustion module and gas turbine were used in the erosion and corrosion evaluation. The combustion tests showed the VFO to match natural gas in flame patterns, temperature profile, and flame color. The operation of the gas turbine revealed that the gas turbine not only operated well on VFO, but its performance was improved. The turbine inlet temperature was lower at a given output with VFO than with either natural gas or diesel fuel. This phenomenon is due to the increase in exhaust mass flow provided by the addition of steam in the diesel for the vaporization process. Following the tests, a thorough inspection was made of materials in the combustion module and on the gas turbine, which came into contact with the vaporized fuel or with the combustion gas. The inspection revealed no harmful effects on any of the components due to the use of VFO. 

Impingement is corrosion caused by aerated water streams constricting metal surfaces. It is similar to erosion corrosion in which air bubbles take the place of particles. The pits formed by impingement attack have a characteristic tear drop shape. 

The information to be compiled about the chemicals, including process intermediates, needs to be comprehensive enough for an accurate assessment of the fire and explosion characteristics, reactivity hazards, the safety and health hazards to workers, and the corrosion and erosion effects on the process equipment and monitoring tools. Current material safety data sheet (MSDS) information can be used to help meet this requirement but must be supplemented with process chemistry information, including runaway reaction and over-pressure hazards, if applicable. 

The rotative speed of a pump is dependent upon the impeller characteristics, type fluid, NPSH available and other factors for its final determination. The most direct method is by reference to manufacturer s performance curves. VVTien a seemingly reasonable selection has been made, the effect of this selected speed on the factors such as NPSH required, suction head or lift, fluid erosion and corrosion, etc., must be evaluated. For many systems these factors are of no concern or consequence. 

D 2809 1989 Test method for cavitation erosion-corrosion characteristics of aluminium pumps with engine coolants... 

Condenser design usually calls for thin-walled tubing with high thermal conductivity characteristics plus a high degree of resistance to biofouling and the many forms of corrosion that may occur. (These forms include crevice and pitting corrosion, biocorrosion, and erosion-corrosion). 

The data to be listed on the Material Technical Sheet needs to be comprehensive enough for an accurate assessment of the fire and explosion characteristics, reactivity hazards, corrosion or erosion effects, and safety, health, and environmental hazards. 

Wear by roll formation is where there is progressive tearing of a layer of rubber which forms a roll. The result is a characteristic abrasion pattern of ridges and grooves at right angles to the direction of movement. The term erosive wear can be applied to the action of particles conveyed in a liquid stream and there can also be corrosive wear due to direct chemical attack of the surface. 

Erosion corrosion is caused by the conjoint action of corrosion and mechanical abrasion by a moving fluid or suspended material in the fluid. Turbulent flow or jets of liquid on a metal surface may lead to erosion corrosion. The mechanical action of the fluid removes the protective corrosion deposit, thus exposing fresh metal to the corrosive. As corrosion products build up, they are removed and so the process continues. The surface of a piece of metal exposed to this type of corrosion has a characteristic structure (Fig. 8). 

---