Boiler corrosion mechanisms

Corrosion mechanisms in boiler plant systems take many forms. They always result in metal wastage and usually result in a loss of mechanical or structural strength as well. 

The corrosion of steel and other metals in a boiler system takes place when an electrochemical cell is established, although the rates of corrosion and the types of corrosion mechanisms involved are highly dependent on the particular circumstances that develop during the operation of individual boiler plants. A failure to adequately control corrosion ultimately leads to the failure of boiler surface components or other components and items of equipment in the system. 

Oxygen is almost always a contributing factor to corrosion mechanisms therefore, the effective removal of dissolved oxygen (DO) is of paramount importance in controlling the rate of boiler system corrosion, irrespective of the size, design, or pressure of the plant. 

Scale is a layer or layers of minerals deposited onto a heat transfer surface, which reduces the heat transfer coefficient (U value, stated in Btu/hr/sq ft/°F). In everyday parlance, scale also refers to thick layers of corrosion product built up onto a metal surface (often present in association with deposits) that may occur at high temperature as a result of a variety of boiler surface corrosion mechanisms. 

Although this chapter looks primarily at relatively common forms of corrosion, we also explore a number of often complex and esoteric boiler section corrosion mechanisms. Many of these types of corrosion will only ever seriously develop in larger high-temperature or high-pressure boilers, especially those requiring knife-edge control and operating under difficult thermal stress conditions. 

Thus, a failure to properly balance and control various external and internal water chemistry parameters may lead to one or more corrosion mechanisms occurring, including several forms involving oxygen. Many types of corrosion are of themselves unfortunately relatively common phenomena, and the mechanisms also are common to all sizes and designs of boiler. 

The precise protocols necessary to achieve effective corrosion control will vary dependent on individual boiler design and operation. For example, control of alkalinity is fundamental in controlling corrosion mechanisms. In small to midsize, general-purpose and industrial boilers, it is common practice to obtain adequate BW alkalinity as part of any water treatment program that operates under a free-caustic regimen. This approach generally is perfectly acceptable, and such programs normally can be relied on to ensure a clean, scale- and corrosion-free boiler. 

At least as important, however, is the need to ensure clean metal surfaces because, as stated earlier, concentration cell corrosion mechanisms commonly occur under boiler section sludges and deposits. 

Exceptions can exist since the corrosion in a wet solution of the interior boiler drum (steel) with dilute caustic soda at high temperature and high pressure and the reaction of high temperature water with aluminum and zirconium have been found to be best interpreted in terms of a dry corrosion mechanism.1... 

Westwood, H.J., Lee, W.K., Corrosion-Fatigue Cracking in Fossil-Fueled Boilers, Corrosion Cracking, Proc. in the International Conf. on Fatigue, Corrosion Cracking, Fracture Mechanics and Failure Analysis, ASM, Salt Lake City, UT, pp. 23-34, 1986. 

Considerable progress has been made in elucidating the mechanisms and causes of boiler corrosion. Advances in this area have been assisted by the development... 

After the "A and B alloys were removed from the autoclave, they were subjected to electrochemical impedance spectroscopy (EIS) measurements under ambient conditions in pH 10 solutions. It was anticipated that such measurements might help to support the corrosion mechanisms hypothesized from the surface analysis of alloys exposed to actual boiler conditions. Analy.sis of the ambient EIS data gave film resistances of 1.2 x 10 and 1.1 x 10 Q-cm 2 for the A and B alloys, respectively, at their corrosion potential. Both are very high resistances, which suggest that a passive film can form on either alloy under these electrochemical conditions. However, in the light of their significantly different behavior under simulated boiler conditions, it can be concluded that these particular EIS tests may not be pertinent to an understanding of the corrosion problem at hand. 

The explicit aims of boiler and feed-water treatment are to minimise corrosion, deposit formation, and carryover of boiler water solutes in steam. Corrosion control is sought primarily by adjustment of the pH and dissolved oxygen concentrations. Thus, the cathodic half-cell reactions of the two common corrosion processes are hindered. The pH is brought to a compromise value, usually just above 9 (at 25°C), so that the tendency for metal dissolution is at a practical minimum for both steel and copper alloys. Similarly, by the removal of dissolved oxygen, by a combination of mechanical and chemical means, the scope for the reduction of oxygen to hydroxyl is severely constrained. 

Deposit control is important because porous deposits, under the influence of heat flux, can induce the development of high concentrations of boiler water solutes far above their normally beneficial bulk values with correspondingly increased corrosion rates. This becomes an increasingly important feature with increase in boiler saturation temperature. In addition, deposits can cause overheating owing to loss of heat transfer. Finally, carryover of boiler water solutes, which can be either mechanical or chemical, can lead to consequential corrosion in the circuit, either on-load or off-load. Material so transported can result in corrosion reactions far from its point of origin, with costly penalties. It is therefore preferably dealt with by a policy of prevention rather than cure. 

The three principal concentration mechanisms postulated as being responsible for on-load corrosion processes by Mann are dry-out, concentration in crevices, and concentration in porous deposits. (Clean boiler tube surfaces on which high-pressure water is boiled under forced convection do not develop concentration factors of more than about two.)... 

Deposit formation, due either to crud (suspended matter-mostly metallic oxides) transported into the boiler or to the products of corrosion in situ, is undesirable as, in many parts of the system, quite apart from the risk of overheating which they present, deposits are able to participate in a mechanism for concentrating solutes to unacceptably high corrosive levels, and are particularly dangerous in high-pressure plant (see Section 4.5). 

JJ..L . Co itioniagaheAimosphere4o Redue AS j 5 .t Qtrssionjafaifaitkm >Erinciples nd. jactiee -, n.3 The Mechanism of Corrosion Prevention by Inhibitors 17.4 Boiler and Feed-water Treatment... 

Today boiler vessels are usually fabricated from special boiler plate and firebox steels of varying thickness, while their auxiliaries (supplementary equipment) and appurtenances (boiler accessories and instruments, especially those employed for safety reasons) may be produced from any of several different constructional metals, alloys, and other materials, including cast iron, copper alloys, stainless steels, and so forth. Tubes and tube plates may be variously constructed of carbon steel, low-alloy steels, or special alloy steels, with each design providing for particular required levels of thermal and mechanical stress and corrosion resistance. The overall boiler plant system may have a life expectancy in excess of 50 to 60 years, although individual components may need to be replaced periodically during this period. 

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