Corrosion under-deposit

A particularly insidious failure mechanism that is commonly found in carbon-steel tubing is under-deposit corrosion. In many cases, corrosion products fomi a scab that can mask the presence of the pitting, making it difficult to quantitatively assess using conventional NDT methods. However, by combining proper cleaning procedures with laser-based inspection methods, the internal surface of the tubing can be accurately characterized and the presence of under-deposit corrosion can be confirmed and quantified. 

Under the sludge considerable orange-red tuberculation corrosion deposits may develop. In cause-and-effect fashion, the tubercles grow cause fouling, permit under-deposit corrosion to persist, and generally act as a binding agent for carbonates, silicates, and other precipitates. 

Deposition commonly reflects a combination of physicochemical processes and localized effects. It may occur through fouling as a result of contamination by process materials, perhaps plus scaling from the supersaturation of dissolved salts, and coupled with some active under-deposit corrosion. As a consequence, deposits forming within a boiler are almost never single mineral scales but typically consist of a variable mix of scale and corrosion debris, chemical treatment residuals, process contaminants, and the like. 

Tuberculation, crevice corrosion, and under-deposit corrosion are... 

Under-Deposit Corrosion In the same way that oxygen becomes depleted in a crevice, and a differential-oxygen concentration cell is established, leading to localized corrosion of the oxygen-starved anodic area, so the same phenomenon readily occurs in dirty boilers under deposits, sludge, and other foulants. 

The rate of metal wastage of this indirect form of corrosion may be increased by the presence of other direct corrosion influences in the deposit or foulant. Also (and similar to crevice corrosion), there may be general oxygen corrosion occurring at the same time or perhaps acting as an initiator to the under-deposit corrosion process. 

This type of corrosion is liable to occur in any part of a boiler where silt, muds, scales, precipitants, or foulants exist and is by no means limited to ferrous metals. Stainless steels, brasses, and cupronickels are all subject to under-deposit corrosion and deep pitting. 

Localized, concentration-cell corrosion (differential aeration corrosion), occurring as Tuberculation corrosion Crevice corrosion Under-deposit corrosion Pitting corrosion All forms of localized, concentration-cell corrosion are indirect attack type corrosion mechanisms. They result in severe metal wastage and can also induce other corrosion mechanisms, e.g. Stress corrosion Corrosion fatigue... 

Blowdown and heat recovery system (BDHR) flash tanks and heat exchangers are potential candidates for sludging, leading to restrictions in the drain line or heat transfer surfaces. Deposits in the BDHR heat exchanger may lead to under-deposit corrosion and leaks. 

Heat exchange surfaces must be kept clean deposits reduce heat transfer efficiency and promote various forms of under-deposit corrosion. It also is easier to keep a clean system clean than to prevent a dirty system from getting dirtier, so measurement of the dirt loading or deposit loading on a heat transfer surface is an important part of determining when a boiler needs cleaning. 

The cooling tower, which is an efficient air scrubber can easily become a catchall for contaminants resulting from the location of the tower or from the industrial process. In arid areas, ingress of sand contributes to fouling, which reduces efficiency and contributes to biofilm and under-deposit corrosion. In coastal areas, sand laden with chlorides can cause corrosion of stainless steel components and impair chemical corrosion inhibitor performance. Heavy industries, such as steel or aluminum manufacture, produce severely contaminated cooling water resulting from direct contact with metal slags and lubricants. 

This demand will reduce the amount of chlorine available for microbiological control and lead to slime growth, especially in the tower basin and water distribution system, with biofilms and under-deposit corrosion being common effects of this problem. 

As discussed above, in high chloride content cooling water, when foulants or deposits are present, the risk of under-deposit corrosion increases, which may be accompanied by a high local chloride concentration. However, it is also likely that a local lowering of pH will occur due to the presence of H+ ions. 

Crevice corrosion, under-deposit corrosion, and tuberculation are all forms of concentration cell corrosion, and all involve oxygen to a greater or lesser extent. 

Pitting corrosion is a general term that can be considered a visible sign of the results of concentration cell corrosion and of further induced-corrosion processes such as when chloride attack occurs. Although pits can also occur with acid corrosion, etc., under-deposit corrosion, of course, can also involve direct metal surface attack, from, say, biologically induced corrosion (but that is discussed separately). 

Galvanic corrosion rates may diminish as corrosion debris begins to act as an electric current insulator (although under-deposit corrosion processes may then take over). 

Because of the ability of glucoheptonates to chelate calcium and iron and dissolve rust films without attacking the bare metal, they are very useful as metal surface cleaners. They are often considered a ferrous corrosion inhibitor, but the real function of these chelants is their ability to dissolve iron- and calcium-rich deposits on the metal surface, within a pH range of 5 to 9, and to provide clean metal surfaces. Thus they permit access by other true corrosion inhibitors and help to minimize differential aeration and under-deposit corrosion mechanisms. 

Chlorine also reacts with common cooling water contaminants, such as hydrogen sulfide, sulfur dioxide, and ammonia. This produces an increase in hydrogen ions (which also tend to lower the system pH) and an increase in total chlorides (which can further increase the risks of corrosion, especially under-deposit corrosion). 

Is the cooling system designed for a large, continuous process application, such as oil, steel, pulp and paper, or petrochemical Is there a risk of oil, ammonia, or sulfur compounds leaking into the system Are there problems of black slime and under-deposit corrosion Who makes the water treatment decisions on-site Is it the laboratory, utilities, production, or all departments collectively Will the customer look for training support ... 

Concentration cell corrosion (crevice corrosion, under-deposit corrosion, tuberculation, pitting corrosion)... 

In discussing environment, we can look at its effect on a macro scale, e.g. in the atmosphere, in the ocean, etc. and also examine effects on a micro scale, i.e. what is happening on the metal surface or over short distances. Due to the great variety of environments in which metals are put to use, the range of corrosion problems are equally numerous. Often, similar types of corrosion occur in many environments and may stem from similar mechanisms these have been given specific names which indicate how the corrosion has occurred. For example, under-deposit corrosion and crevice corrosion are related, both being due to oxygen concentration cells. 

Under-deposit corrosion is a particular type of corrosion caused by differential aeration. If sparingly soluble salts, loosely adherent corrosion products, algal, or other fouling, is deposited on a metal surface, then these areas become depleted in oxygen. Unfouled or less fouled areas have a greater supply of oxygen and hence become cathodic to the fouled areas. Thus, the anodic under-deposit areas will corrode preferentially. 

Fig. 5. Advance of under-deposit corrosion on a heat exchanger tube. A, Anodic area B, corrosion products C, cathodic area.

Fig. 6. Crevice corrosion beneath a washer. Note the under-deposit corrosion due to seepage of corrosion products from the crevice.

In fact, when testing corrosion inhibitors, the major amount of corrosion may occur under the mounting washers as fresh inhibitor cannot reach such a confined space and, in addition, the low oxygen concentration leads to corrosion. As in the section above, ferric ions may seep from within the crevice and deposit around the fixture, thus giving rise to under-deposit corrosion. 

Aluminum and alloys are not suitable for (1) alkalis, (2) acids at pH 4.5, and (3) mercury, which can be a significant risk in some liquified natural gas operations. The heat treatable, high-strength aluminum alloys of the 2000- and 7000-series are rarely used because of environmental cracking susceptibility. Aluminum and its alloys are susceptible to chloride pitting and to concentration cell problems such as crevice corrosion and under-deposit corrosion. 

For crevices such as in those in socket welds, the metal in the crevice is likely to be anodic. Crevice corrosion and under-deposit corrosion can be serious problems in oxide-stabilized materials such as aluminum and the stainless steels. Crevices and deposits can also accelerate corrosion in metals (such as carbon steel) that do not exhibit both active and passive states. However, the rate of corrosion is much slower in such materials because they lack the galvanic driving force of the active-passive states characteristic of the oxide-stabilized metals and alloys. The anode areas in crevices and under deposits are typically smaller than the cathode areas. This difference accelerates the corrosion rate. 

Carbon dioxide (COj) corrosion Hydrogen sulfide (HjS) corrosion Preferential weld corrosion Erosion and erosion-corrosion Crevice corrosion Flange face corrosion Cavitation Dead-leg corrosion Under-deposit corrosion Microbial corrosion Oxygen corrosion Galvanic corrosion External corrosion Corrosion under insulation (CUI)... 

Y. J. Tan, Y. Fwu and K. Bhardwaj, Electrochemical evaluation of under-deposit corrosion and its inhibition using the wire beam electrode method , Corros. Sci, 53, 1254(2011). 

NACE Task Group TG 380, Under-deposit Corrosion testing and mitigation methods, NACE, 2009. 

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