Deposit attack

Underdeposit corrosion is not so much a single corrosion mechanism as it is a generic description of wastage beneath deposits. Attack may appear much the same beneath silt, precipitates, metal oxides, and debris. Differential oxygen concentration cell corrosion may appear much the same beneath all kinds of deposits. However, when deposits tend to directly interact with metal surfaces, attack is easier to recognize. 

Deposit attack occurs when there is nonuniform deposition of a film on a metal surface. The most common form appears as unequal scale deposits in an aqueous environment. Unequal film provides for concentration cells, which degrade the metal by galvanic means. 

Deposit Attack—to localized corrosion under, and resulting from, a deposit on a metal surface. 

Oxidation and tarnishing active dissoiution in acids anodic oxidation and passivity chemicai and eiectrochemical polishing atmospheric and immersed corrosion in certain cases Crevice corrosion hiiform corrosion deposit attack bimetaliic corrosion intergranuiar corrosion weid decay... 

Deposits of sand, dirt or permeable corrosion products on the metal surface (a type of crevice corrosion that is referred to as deposit attack). 

Deposit attack and pitting When water speeds are low and deposits settle on the surface (particularly at water speeds below about 1 m/s), pitting of copper and copper alloys is liable to occur by differential aeration effects. 

Stainless steel also is susceptible to crevice corrosion and deposit attack. Differential aeration cells, formed between stagnant and well-aerated areas on the metal surface, may promote rapid attack. 

St. Thomas Church in New York City. Areas on the facade accumulate a layer of gypsum (calcium sulfate) produced by acid deposition attack on the limestone and then darken by dirt, soot, and other combustion products. (Photo by Elena... 

The addition of 1 to 2 mass% Fe was shown to improve the corrosion resistance impingement attack, deposit attack, pitting, of copper alloys containing 10 mass% Ni [1951Bai]. Copper precipitation has been... 

Precipitates and suspended solids in process streams will deposit in stagnant areas (e.g., comers in vessels or bends in a pipe) and lead to corrosion of metals by the same mechanism as that involved in crevice corrosion. This is called deposit attack. Filtration of process fluids can prevent these problems. 

Deposit attack n. Excessive corrosion that may occur under solid substances that... 

Under-deposit attack or poultice corrosion may occur when a metal is locally covered by foreign, absorbent (organic or inorganic) materials [40,45]. In this case, attack can proceed even when the bulk of the system is dry due to retention of moisture in the poultice. The corrosion mechanism is similar to crevice corrosion in that the deposits act to limit the migration of oxygen to the covered area. This leads to acidic shifts in pH, concentration of Cl ions in the shielded area, and a shift to a more active corrosion potential under the deposit. Local corrosion rates can be very high due to the large cathode-to-anode area ratio. 

Prmide suitable strainers in lines to prevent local olmtruc-lion within the equipment wdiich may start deposit attacks or ppHfilt in impingement attack. 

Deposit Attack Excessive corrosion that may occur under sohd substances that may be deposited on a metal surface and thus shield it from ready access to oxygen or ions in the solution. This is a form of concentration cell corrosion. 

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