Acid attacks

The hydrated components (C-S-H, portlandite, sulfoaluminates) in the cement matrix of concrete are in equilibrium with the pore liquid that is characterized by a high pH, due to the presence of OH (balanced by Na and K ). When concrete comes into contact with acid solutions, these compounds may dissolve at a rate that depends on the permeability of the concrete, the concentration and the type of acid. In soil with acidic ground water, the rate of refreshing is important Acids that can attack concrete are sulfuric acid, hydrochloric acid, nitric acid, organic acids such as acetic acid and humic adds and solutions of CO2. The rate of attack on the cement matrix depends on the solubility of the salts that are formed 

The aggressive chemical environments classified below are based on natural soil and ground water at water/soil temperatures between 5-25°C and a water velocity sufficiently low to approximate to static conditions.The most onerous value for any single chemical characteristic determines the class.Where two or more aggressive characteristics lead to the same class, the environment shall be classified into the next higher class, unless a special study for this specific case proves that it is not necessary.  

Clay soils with a permeability below 10 m/s may be moved into a lower class. 

The test method prescribes the extraction of S04 by hydrochloric acid alternatively, water extraction may be used, if experience is available in the place of use of the concrete. 

The 3000 mg/kg limit is reduced to 2000 mg/kg where there is a risk of accumulation of sulfate ions in the concrete due to drying and wetting cydes or capillary suction. 

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Diamond is very resistant to chemical reagents potassium dichromate and sulphuric acid attack it with the formation of CO2. It burns in air or oxygen at 700 C to CO2 leaving scarcely any ash some forms of bort may leave as much as 4-5% ash. 

Factors other tlian tire Si/Al ratio are also important. The alkali-fonn of zeolites, for instance, is per se not susceptible to hydrolysis of tire Al-0 bond by steam or acid attack. The concurrent ion exchange for protons, however, creates Bronsted acid sites whose AlO tetraliedron can be hydrolysed (e.g. leading to complete dissolution of NaA zeolite in acidic aqueous solutions). 

Strong oxidising acids, for example hot concentrated sulphuric acid and nitric acid, attack finely divided boron to give boric acid H3CO3. The metallic elements behave much as expected, the metal being oxidised whilst the acid is reduced. Bulk aluminium, however, is rendered passive by both dilute and concentrated nitric acid and no action occurs the passivity is due to the formation of an impervious oxide layer. Finely divided aluminium does dissolve slowly when heated in concentrated nitric acid. 

In addition to the abnormal properties already discussed, aqueous hydrofluoric acid has the properties of a typical acid, attacking metals with the evolution of hydrogen and dissolving most metallic hydroxides and carbonates. 

E values. Nitric acid attacks mercury, oxidising it to Hg (aq) when the acid is concentrated and in excess, and to Hgf (aq) when mercury is in excess and the acid dilute. Hydriodic acid Hl(aq) attacks mercury, because mercury(Il) readily forms iodo-complexes (see below, p. 438). 

Alkali solutions and dilute and concentrated acids attack the metal rapidly. The pure metal is likely to ignite if scratched with a knife. 

Copolymer. Acetal copolymers are prepared by copolymerization of 1,3,5-trioxane with small amounts of a comonomer. Carbon-carbon bonds are distributed randomly in the polymer chain. These carbon-carbon bonds help to stabilize the polymer against thermal, oxidative, and acidic attack. 

Uses. Lithium fluoride is used primarily in the ceramic industry to reduce firing temperatures and improve resistance to thermal shock, abrasion, and acid attack (see Ceramics). Another use of LiF is in flux compositions with other fluorides, chlorides, and borates for metal joining (17) (see Solders). 

The Zinc Phosphating Process. The zinc phosphating reaction involves acid attack on the substrate metal at microanodes and deposition of phosphate crystals at microcathodes (8). Liberation of hydrogen and the formation of phosphate sludge also occur. The equation for the dissolution of iron together with precipitation of dissolved iron as sludge in a nitrite accelerated system is as foUows ... 

Reclaim Rubber. The process of reclaiming mbber by chemical digestion has been in use since the late 1800s. Early processes involved the treatment of the mbber cord mix with acid. Acids attack cotton, rayon, and nylon. The acid treatment was used to remove the reinforcing components. 

Tin does not react directly with nitrogen, hydrogen, carbon dioxide, or gaseous ammonia. Sulfur dioxide, when moist, attacks tin. Chlorine, bromine, and iodine readily react with tin with fluorine, the action is slow at room temperature. The halogen acids attack tin, particularly when hot and concentrated. Hot sulfuric acid dissolves tin, especially in the presence of oxidizers. Although cold nitric acid attacks tin only slowly, hot concentrated nitric acid converts it to an insoluble hydrated stannic oxide. Sulfurous, chlorosulfuric, and pyrosulfiiric acids react rapidly with tin. Phosphoric acid dissolves tin less readily than the other mineral acids. Organic acids such as lactic, citric, tartaric, and oxaUc attack tin slowly in the presence of air or oxidizing substances. 

High Pressure Boiler Water Treatment. High pressure boilers usually have feed water composed of demineralized makeup water and a high percentage of condensate returns. Because of these conditions, high pressure boilers are prone to caustic and acid attack. Low pressure boilers that use dernineralized water and condensate as feed water are also susceptible to caustic and acid attack. 

MetaUic arsenic is not readily attacked by water, alkaline solutions, or nonoxidizing acids. It reacts with concentrated nitric acid to form orthoarsenic acid [7778-39-4] H AsO. Hydrochloric acid attacks arsenic only in the presence of an oxidant. 

BeryUium reacts readUy with sulfuric, hydrochloric, and hydrofluoric acids. DUute nitric acid attacks the metal slowly, whereas concentrated nitric acid has Httle effect. Hot concentrated alkaUes give hydrogen and the amphoteric beryUium hydroxide [13327-32-7] Be(OH)2. Unlike the aluminates, the beryUates are hydrolyzed at the boU. 

Chlorosulfuric acid attacks brass, bronze, lead, and most other nonferrous metals. From a corrosion standpoint, carbon steel and cast Hon are acceptable below 35°C provided color and Hon content is not a concern. Stainless steels (300-series) and certain aluminum alloys are acceptable materials of constmction, as is HasteUoy. Glass, glass-lined steel, or Teflon-lined piping and equipment are the preferred materials at elevated temperatures and/or high velocities or where trace Hon contamination is a problem, such as in the synthetic detergent industry. 

ABS plastic, a polymer consisting of polybutadiene spheroids is dispersed in a continuous phase of poly(styrene—acrylonitrile). The chromic acid attacks the polybutadiene at a much higher rate than the continuous phase. This gives an excellent microroughened surface with superior metal-to-plastic bond strength. A typical recommended formulation consists of 20 vol % sulfuric acid, 420 g/L chromic acid, and 0.1—1.0% of a fluorocarbon wetting agent. The plastic is treated with this formulation for 6—10 min at 60—65°C. 

Resistance to Chemical Attack. The resistance to alkah and acid attack is evaluated on the basis of loss in weight, loss in gloss, or reduced cleanabihty of the surface. 

Aluminum has high resistance to atmospheric conditions as well as to industrial fumes and vapors and fresh, brackish, or salt waters. Many mineral acids attack aluminum, although the metal can be used with concentrated nitric acid (above 82 percent) and glacial acetic acid. Aluminum cannot be used with strong caustic solutions. 

This case history illustrates the paradox so often encountered in microbiologically influenced corrosion. Clearly, two corrosion mechanisms were operating in the system, namely, acid attack and microbiologically influenced corrosion. To what degree each mechanism contributed to wastage is difficult to quantify after the fact. This was especially the case here, since other areas of the rolling oil system were attacked by a predominantly acidic form of corrosion. 

Acidic attack on stainless steels differs from corrosion on nonsteunless steels in two important respects. First, nonoxidizing acid corrosion is usually more severe in deaerated solutions second, oxidizing acids attack stainless steel far less strongly than carbon steel. Hence, nitric acid solutions at low temperatures cause only superficial damage, but hydrochloric acid causes truly catastrophic damage. 

Fresh acid attack is recognized by the absence of corrosion product in wasted areas and the sharpness of attack. Oxide layers are usually easily stripped by a test drop of hydrochloric acid in freshly corroded areas. Deposits are almost always absent. Edges of attacked areas are sharp and angular, as intervening corrosion has not recently occurred. In stainless steels such distinctions blur, as corrosion in intervening periods is usually slight. 

Figure 7.4 Cross section through a mild steel pipe wall suffering severe acid attack.

Weld attack. Welds are often more susceptible to corrosion than other areas (see Chap. 15, Welds Defects ). Welds may contain porosity, crevices, high residual stresses, and other imperfections that favor attack. Carbon steel welds are usually ditched by acid attack (Fig. 7.10). 

Cavitation produces spongy, porous-appearing surfaces, strongly resembling acid attack. However, cavitation usually causes highly localized areas of metal loss, unlike acid, which attacks over a much wider area. 

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