Caustic corrosion mechanism

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. 

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... 

As suggested in the mechanism of caustic corrosion, caustic in a concentrated form dissolves the magnetic oxide layer on the tube wall. 

Corrosion also occurs as a result of the conjoint action of physical processes and chemical or electrochemical reactions (1 3). The specific manifestation of corrosion is deterrnined by the physical processes involved. Environmentally induced cracking (EIC) is the failure of a metal in a corrosive environment and under a mechanical stress. The observed cracking and subsequent failure would not occur from either the mechanical stress or the corrosive environment alone. Specific chemical agents cause particular metals to undergo EIC, and mechanical failure occurs below the normal strength (5aeld stress) of the metal. Examples are the failure of brasses in ammonia environments and stainless steels in chloride or caustic environments. 

Alloy 400 has good mechanical properties and is easy to fabricate in all wrought forms and castings. K-500 is a modified version of this alloy and can be thermally treated and is suitable for items requiring strength, as well as corrosion resistance. Alloy 400 has immunity to stress corrosion cracking and pitting in chlorides and caustic alkali solutions. 

Selective removal of the less noble constituent has been demonstrated by chemical analysis in the case of nickel-rich alloys in fused caustic soda or fused fluorides ", and by etching effects and X-ray microanalysis for Fe-18Cr-8Ni steels in fused alkali chlorides. This type of excessive damage can occur with quite small total amounts of corrosion, and in this sense its effect on the mechanical properties of the alloy is comparable with the notorious effect of intercrystalline disintegration in the stainless steels. 

Where caustic deposits occur, the resultant corrosion of steel by caustic gouging or stress corrosion cracking (SCC) mechanisms produces particulate iron oxides of hematite and magnetite. It is common to see white rings of deposited sodium hydroxide around the area of iron oxide formation. 

In practice, the potential causes of boiler section corrosion are many and often commonplace. Initiators include oxygen, carbon dioxide, acid, caustic, copper plating, chelant, and even the water itself. In addition, mechanical problems may be an initiator of corrosion, which in turn may lead to boiler mechanical failure. 

Nickel has good mechanical properties and is easily worked. The pure metal (>99 per cent) is not generally used for chemical plant, its alloys being preferred for most applications. The main use is for equipment handling caustic alkalies at temperatures above that at which carbon steel could be used above 70°C. Nickel is not subject to corrosion cracking like stainless steel. 

Oxides are always present on the surface of transition metals in alkaline solution. At open circuit they are intermediates in the mechanism of corrosion. The resistance of Ni towards corrosion in base is better than Fe or mild steel, especially at high caustic concentration and high temperature [23, 24]. The role of surface oxides in the cathodic range of potentials depends on the conditions of their formation. Thus, a reducible layer of hydroxide Ni(OH)2 or even oxohydroxide NiOOH has been found [385] to be beneficial for the electrocatalytic activity. It has even been claimed [386] that some good performances are specifically due to the formation of oxide layers during the preparation (Fig. 19). An activation of the Ni surface by the application of anodic current pulses has been reported [387] to be beneficial owing to the formation of Ni(OH)2 layers. This has been confirmed by impedance studies of the mechanism [388]. 

I. Mechanism of toxicity. Ammonia gas is highly water-soluble and rapidly produces an alkaline corrosive effect on contact with moist tissues such as the eyes and upper respiratory tract. Exposure to aqueous solutions causes corrosive alkaline injury to the eyes, skin, or gastrointestinal tract (see Caustic and Corrosive Agents, p 157). 

Chlorides were involved mainly in the cracking of stainless steels, and caustics and nitrates were responsible for the cracking of steel. The studies reported that a 50-50 distribution between mechanical and corrosion failures was found to hold good. 

These terms are evidently ambiguous for example, it is not always clear whether wet is confined to aqueous solutions—the wetting of solids by mercury indicates that liquid-metal corrosion should be classified as wet . Even if the term is restricted to aqueous solutions, the difficulty arises that the mechanism of growth of magnetite scale during the reaction of the interior of a boiler drum with dilute caustic soda at high temperatures and pressures is best interpreted in terms of a dry corrosion process. Similar considerations apply to the reactions of aluminium and zirconium with high-temperature water. 

Regardless of how attractive a material may be from any other point of view, it is of no use for a particular purpose if it cannot be secured in the required form. Filter cloth cannot be woven from an alloy available only as castings. Several materials may possess the corrosion resistant and mechanical properties required for a job, but many of them may be too expensive to be considered. For example, silver might be somewhat better than nickel for tubes in an evaporator to concentrate caustic soda to 50 percent, but it would not be enough to justify the extra cost involved, and steel might be a better choice economically overall for handling dilute caustic under less stringent conditions [4]. 

The most outstanding mechanism which has found a wide acceptance is the film rupture mechanism. This mechanism has been extensively studied in stress corrosion cracking of alpha-brass in ammoniacal environment, although it was originally proposed for caustic cracking of boiler steel. Here are some salient features of the mechanism. The items are illustrated in Fig. 4.43. 

Austenitic nickel-chromium stainless steels and mild steel are subject to stress corrosion cracking in caustic soda (caustic cracking) at elevated temperatures. The phenomenon, caustic cracking is mostly encountered in boilers. Caustic is added as an additive to boiler water in order to preserve the thin film of magnetic iron oxide by raising the pH. Caustic addition creates problems only when it becomes concentrated. It can become concentrated by one of the mechanisms summarized below. Four instants in the life of a steam bubble are shown in Fig. 4.58. 

Since phosphoric acid is essentially nonvolatile, thermal decomposition of diammonium phosphate in solution yields only ammonia gas in the vapor phase. The phosphate ion is extremely stable and is not altocd in the process therefore, no blowdown is required and only a small makeup of phosphoric acid is needed to compensate for imavoidable mechanical losses. The small amount of acid gases, acidic organics, and neutral compounds that are co-absorbed widi the ammonia are either recycled to the absorber or reacted with caustic soda to prevent corrosion and ammonia product contamination in the fractionator. 

PPS offers a combination of high temperature stability, excellent mechanical strength and dimensional stability, along with resistance to corrosion by common solvents, caustic solutions, and dilute acids. The material is challenging metals, thermosets, or other engineering plastics in many domestic appliance applications. 

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