Corrosivity and causticity

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. 

Corrosivity and Causticity The pH, if applicable, of the solvation media should be less than 11.0 but greater than 2.5. 

The replacement of many hazardous and toxic materials currently used in water, e.g. toxic form of chromium(VI), cyanide, highly corrosive and caustic electrolytes, would save about 10% of the current treatment costs. 

Most alkoxides are highly basic. These moisture-sensitive substances are corrosive and caustic. Skin contact can cause irritation. However, highly exothermic water-reactive compounds such as potassium tert-butoxide, are strongly corrosive and can cause bums. 

The concentrated solution of potassium hydroxide is highly corrosive and caustic. Do not allow it to come into contact with skin. Should it do so, flood the affected area immediately with water and then thoroughly rinse it with 1% acetic acid. Wear latex gloves when preparing and transferring solutions of potassium hydroxide. 

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). 

Protective measures against corrosive and caustic effects of the electrolyte, e.g. sulfuric acid (H2SO4) in lead-acid batteries and potassium hydroxide (KOH) in NiCd batteries. 

Steam Generators iBtacks in the weU between the tube shell and tube plate caused by corrosion and caustic fragility due to incorrect treatment of water Pemolition ... 

A first operation on the crude, desalting (washing by water and caustic), extracts salts (NaCl, KCl and the MgCb that is cdn eft4rdJt6 NaCl by the caustic), reduces acid corrosion as well as it minimizes fouling and deposits. /... 

This secondary reaction starts at about 180°C, but the mass must be heated to 350—400°C to bring the reaction to completion and produce a nitrate-free product. The off-gases are extremely corrosive and poisonous, and considerable attention and expense is required for equipment maintenance and caustic-wash absorption towers. Treatment of the alkaline wash Hquor for removal of mercury is required both for economic reasons and to comply with governmental regulations pertaining to mercury ia plant effluents. 

Acids such as sulfuric, hydrochloric, nitric, and especially hydrofluoric as well as strong alkaUes such as caustic soda and caustic potash are extremely corrosive to animal and vegetable tissue. Extreme caution must be taken to prevent skin contact, inhalation, or ingestion. Violent reactions may occur when dissolving or diluting many of these chemicals with water. 

Health and Safety. Protective clothing that is compatible with the remover formula must be worn. Caustic soda baths should be ventilated to remove vapors from the work area. Most caustic removers are corrosive and cause severe bums with minimal contact to the skin. Canister respirators that are compatible with the remover should be worn. 

Alkali Treatment. Caustic washing is the treatment of materials, usually products from petroleum refining, with solutions of caustic soda. The process consists of mixing a water solution of lye (sodium hydroxide or caustic soda) with a petroleum fraction. The treatment is carried out as soon as possible after the petroleum fraction is distilled, since contact with air forms free sulfur, which is corrosive and difficult to remove. The lye reacts either with any hydrogen sulfide present to form sodium sulfide, which is soluble in water, or with mercaptans, foUowed by oxidation, to form the less nocuous disulfides. 

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

Virtuallv evety alloy system has its specific environment conditions which will prodiice stress-corrosion cracking, and the time of exposure required to produce failure will vary from minutes to years. Typical examples include cracking of cold-formed brass in ammonia environments, cracking of austenitic stainless steels in the presence of chlorides, cracking of Monel in hydrofluosihcic acid, and caustic embrittlement cracking of steel in caustic solutions. 

Carbon steel is easily the most commonly used material in process plants despite its somewhat limited corrosion resistance. It is routinely used for most organic chemicals and neutral or basic aqueous solutions at moderate temperatures. It is also used routinely for the storage of concentrated sulfuric acid and caustic soda [up to 50 percent and 55°C (I30°F)]. Because of its availability, low cost, and ease of fabrication steel is frequently used in services with corrosion rates of 0.13 to 0.5 mm/y (5 to 20 mils/y), with added thickness (corrosion allowance) to assure the achievement of desired service life. Product quahty requirements must be considered in such cases. 

When heat stable salt buildup becomes a problem a variety of options may manage it. These include partial or total solution replacement, heat stable salt removal, or adding caustic to neutralize the heat stable salts. Many operators choose caustic addition because it is perceived to be a more economical way to stop corrosion and subsequent foaming and loss problems. 

Tests to simulate real-world amine plant operations have shown that caustic addition doesn t substantially improve solution corrosivity and in some cases corrosion rates increase. Maintenance of low heat stable salt anion levels is a better way to go. Concentrations as low as 250 ppm are encouraged and 5,000 to 8,000 ppm seem to be tolerable. Caustie doesn t reduce the heat stable salt content of amine solution. 

Embrittlement embrittlement and for improperly heat treated steel, both of which give intergranular cracks. (Intercrystalline penetration by molten metals is also considered SCC). Other steels in caustic nitrates and some chloride solutions. Brass in aqueous ammonia and sulfur dioxide. physical environments. bases of small corrosion pits, and cracks form with vicious circle of additional corrosion and further crack propagation until failure occurs. Stresses may be dynamic, static, or residual. stress relieve susceptible materials. Consider the new superaustenitic stainless steels. 

The EPA defines corrosivity in terms of pH (i.e., wastes with pH <2 or >2.5) or in terms of ability to corrode steel (SAE 20) at a rate of >6.35 mm (0.250 in.) per year at a temperature of 55 C (13°F). This discussion will address corrosivity as it applies to acids and caustics. Acids are compounds that yield H ions (actually HjO ions) when dissolved in water. Common industrial acids include acetic, nitric, hydrochloric, and sulfuric acids. The terms concentrated and dilute refer to the concentrations in solution. Mixing a concentrated acid with enough water will produce a dilute acid. For example, a bottle of concentrated HCl direct from the manufacturer is approximately 12 N in HCl, while a solution of HCl used in a titration may be only 0.5 N. The latter is a dilute acid solution. 

Chemical Reactivity - Reactivity with Water No reaction Reactivity with Common Materials Corrosive, particularly when diluted. Attacks most common metals, including most stainless steels. Excellent solvent for many synthetic resins or rubber Stability During Transport Stable Neutralizing Agents for Acids and Caustics Dilute with water, rinse with sodium bicarbonate solution Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

Chemical Reactivity - Reactivity with Water Slow reaction with water to produce hydrochloric acid fumes. The reaction is more rapid with steam Reactivity with Common Materials Slow corrosion of metals but no immediate danger Stability During Transport Not pertinent Neutralizing Agents for Acids and Caustics Soda ash and water, lime Polymerization Does not occur Inhibitor of Polymerization Not pertinent. 

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