Caustic corrosion

Caustic corrosion (gouging) occurs when caustic is concentrated and dissolves the protective magnetite (Fe O layer. Iron, in contact with the boiler water, forms magnetite and the protective layer is continuously restored. However, as long as a high caustic concentration exists, the magnetite is constantiy dissolved, causing a loss of base metal and eventual failure (Fig. 4). 

This results in the prevention of caustic buildup beneath deposits or within a crevice where leakage is occurring. Caustic corrosion (and caustic embrittlement, discussed later) does not occur, because high caustic concentrations do not develop. 

Fig. 5. Coordinated phosphate—pH control avoids both acid and caustic corrosion. To convert psi to kPa, divide by 6.895.

Atz. caustic, corrosive (Calico) discbarging, discharge, -alkab, n. caustic alkali, -alka-lildsung, /. caustic alkaline solution. 

Where a deposit contains an adequate concentration of sodium hydroxide and the affected area is stressed to a sufficiently high level, stress-corrosion cracking or caustic embrittlement (SCC) may occur. This type of caustic corrosion is different from caustic gouging, which does not require the presence of stress. 

Caustic gouging usually occurs only in areas of high heat flux but may also result when heat transfer rates are low, as in horizontal or inclined WT boiler tubes under circumstances in which the steam-water velocity is particularly low. Here, the relatively small volume of BW surrounding the steam bubbles concentrates very quickly, the alkalinity soars, and caustic corrosion develops. 

In addition to stress, the development of caustic embrittlement requires a caustic corrosive in a concentration of at least 7.5 to 10% NaOH. It also requires some silica to be present (typically = 0.2%). Thus, the prevention of this form of corrosion primarily requires the elimination of either stress or the concentration of corrosive. 

Romero, Matilde F. Pdrez, Orlando Navarro, Alfredo. Phosphate Attack as Caustic Corrosion in High-Pressure Boiler Tubes. Material Performance . NACE International, USA, March 1999. 

Sodium hydroxide (NaOFI), also called lye, soda lye, or caustic soda to distinguish it from potassium hydroxide (potash lye), is another important base. Flistorically, lye was obtained from the ashes of wood and used to make soap. Lye, however, is an extremely caustic chemical. It can cause serious chemical burns if it comes into contact with the skin and permanent blindness if it gets into the eyes. People had to be very careful while making the soap. They also had to make sure they got the mixture of lye and animal fat (lard) correct to keep from hurting themselves and their families. Because of its caustic (corrosive) nature, sodium hydroxide is also used as the active ingredient in oven and drain cleaners. 

Both sodium and potassium salts are natural constituents of body tissues and fluids as Na+ and K+ ions, respectively, and are not themselves toxic at normal physiological levels. The oxides and hydroxides of both these metals are very caustic, corrosive substances that damage exposed tissue. Oxides are formed by the combustion of sodium and potassium organometallics, and hydroxides are produced by the reaction of the oxides with water or by direct reaction of the organometallics with water, as shown below for cyclopentadienylsodium ... 

Corrosion General corrosion, crevice corrosion, pitting, hydrogen damage, caustic corrosion, intergranular corrosion, microbiological corrosion... 

Caustic Corrosion. Concentration of caustic (NaOH) can occur either as a result of steam blanketing (which allows salts to concentrate on boiler metal surfaces) or by localized boiling beneath porous deposits on tube surfaces. 

Nickel alloys plus nickel are widely used for services, including acids, caustics, corrosive waters, and for numerous corrosive process applications and low- and high-temperature applications. For further information contact ... 

Contents Include pulmonary agents, heavy metals, organics, caustics, corrosives, strong oxldents and reducing agents, microbial hazards, human reproduction and Chemical exposure, and safety programs. 

Precaution Caustic corrosive powerful acidic oxidizer ignites or explodes on contact with many materials reacts with water to produce heat reactive with oxidizing/reducing materials Hazardous Decomp. Prods. Heated to decomp., emits toxic fumes of SO,... 

Because of the high reactivity of sodium with air and caustic corrosion risks that may arise during repair and further operations, due to sodium/moist air reaction products, residual sodium had to be eliminated in order to achieve the requalification of the SG before start up. For this, a particular sodium cleaning procedure has been established, qualified and industrially implemented in a very short delay in accordance with Phenix start up planning. 

The caustic corrosion risks must be limited at every step ... 

Bases have a bitter taste and a slippery, soapy feel. Often the terms alkali, alkaline, lye, and caustic are associated with bases. Sodium hydroxide and potassium hydroxide have, for example, been referred to as caustic soda and caustic potash, respectively. The reason is that these compounds are caustic, corrosive compounds that can chemically attack a variety of materials, including plant and animal tissue. These strong bases are used in the production of soaps. 

Kationenaustauscher (starker/schwacher) cationic poiymerization kationische Polymerisation cauiifiower poiymer Blumenkohlpolymer caustic (corrosive/mordant) atzend, beizend, koirosiv caustic agent Atzmittel caustic hazard Veratzungsgefahr caustic lime CaO Branntkalk caustic potash/ potassium hydroxide KOH Atzkali, Kaliumhydroxid... 

Caustic corrosion of unalloyed and low-alloy steel is encountered in some unusual situations. For example, in boilers traces of sodium hydroxide can become concentrated and cause local corrosion and caustic embrittlement. This occurs usually in boiler tubes that alternate between wet and dry conditions or in which deposits form. Boiler feed water permeates the deposits and evaporates. This causes concentration of the caustic material, to up to several percent, which is enough to destroy the protective magnetite and/or to initiate caustic embrittlement (Effertz et al., 1982 Hersleb, 1982). 

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

As the microcracks accumulate, the tube strength decreases. The tube bursts when the internal pressure caused by the accumulation of gas exceeds the yield strength of the metal which is still intact. The crack is longitudinal and it resembles the crack caused by creep rupture. The methods applied to control caustic corrosion are also valid for controlling hydrogen damage. 

Caustic corrosion Corrosion resulting from the concentration of caustic soda and dissolution of the protective magnetite layer. 

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