Steel alkaline corrosion

Addition of nickel improves the resistance of iron and steel to corrosion by alkaline solutions. The beneficial effect is most pronounced in hot, strong caustic solutions as illustrated by the results on nickel cast irons in Table 3.37. 

The coating is applied to protect the steel from corrosion due to the acid or alkaline condition of the soil surrounding the pipe in service. Usually, the process requires three layers. First, an epoxy powder is applied to achieve adhesion to the pretreated metal and therefore resistance to cathodic disbondment. Second, a tie layer of polyolefin copolymer is applied and third a thick layer of polyethylene is cascaded, which in effect protects the epoxy from physical damage. 

Active metals such as aluminum, titanium, and high-chromium steels become corrosion resistant under oxidizing conditions because of a very adherent and impervious surface oxide film that, although one molecule thick, develops on the surface of the metal. This film is stable in a neutral medium, but it dissolves in an acid or alkaline environment. In a few cases, such as certain acid concentrations, metals can be kept passive by applying a carefully controlled potential that favors the formation of the passive surface film. The ability to keep the desired potential over the entire structure is very critical in anodic control. If a higher or lower potential is applied, the metal will corrode at a higher rate, possibly higher than if it is not protected at all. 

The alkaline nature of the environment surrounding the reinforcement steel rods embedded in the concrete matrix passivates the steel however, corrosion attack takes place when chloride ions penetrate into them from the seawater and other surroundings [64, 65]. The other ions like sulfate and carbonate ions also enhance the possibility of corrosion of the steel rods. Cathodic protection of such steel rods can be classified into two categories the surrounding of the steel rod is the only concrete environment and the other is any medium encountered by the structure. 

Wiens, U., Breier, W., and Schiessel, P. (1995) Influence of high silica fume and high fly ash contents on alkalinity ofpore solution and protection of steel against corrosion. American Concrete Institute SP-153 (Vol. 2), pp. 749-761. 

Normally, reinforcing steel is in a passive state due to the formation of a tenacious passive oxide coating called gamma ferric oxide on the steel surface created by the highly alkaline environment (pH > 12.5) of concrete. However, the presence of chloride ions at the concrete/steel interface in excess of the reaction threshold level 0.5 kg to 1.0 kg of chloride ions/meter concrete (1.0-2.0 lb of chloride ions/yard ) depassivates the steel, and corrosion will initiate. On bridges, the source of the chloride ion is usually deicing chemicals applied in the snow-belt areas in winter or salt spray with seawater in coastal areas. 

The electrons liberated in the anodic reaction (Eq. (5 a)) have to be consumed immediately by the cathodic reaction (Eq. (5 b)) (condition of electroneutrality), thus both reactions have to take place with the same reaction rate. In the atmosphere or in soil, the corrosion product Fe(OH)2 is further oxidized and forms rust in the highly alkaline pore solution in concrete the same reaction forms a protective, very thin oxide film on the steel and corrosion stops. 

Haynes G. In Corrosion Tests and Standards Apphcation and Interpretation, Baboian R., ed. ASTM Manual Series MNL 20, Philadelphia, PA, 19103, 1995, pp. 91-97 Hirai S., Shimakagen K., Aizawa S. Alkaline corrosion resistance of anodised coated with zirconium oxide by a sol-gel process. J. Am. Ceram. Soc. 1998 81(12) 3087-3092 Hirayama R., Harayama S. Electrochemical impedance for degrades coated steel having pores. Corrosion 1991 47(12) 952-963... 

In general, the presence of oxygen or of acidic conditions promotes the corrosion of carbon steel. Alkaline conditions inhibit corrosion. Factors that affect the corrosion resistance of these steels are ... 

These reactions would result in the formation of oxide at cathodic sites. The high alkalinity of cement paste, however, provides protection for the steel reinforcement. Corrosion is increased in the presence of chloride salts. It contributes, together with COj ingress, to the depression of the pH of the pore fluid and increases the eleetrieal conductance of the concrete, allowing the corrosion current to increase. 

Dichloroethylene is usually shipped ia 208-L (55 gal) and 112-L (30 gal) steel dmms. Because of the corrosive products of decomposition, inhibitors are required for storage. The stabilized grades of the isomers can be used or stored ia contact with most common constmction materials, such as steel or black iron. Contact with copper or its alloys and with hot alkaline solutions should be avoided to preclude possible formation of explosive monochloroacetylene. The isomers do have explosive limits ia air (Table 1). However, the Hquid, even hot, bums with a very cool flame which self-extiaguishes unless the temperature is well above the flash poiat. A red label is required for shipping 1,2-dichloroethylene. 

The addition of small amounts of alloying materials greatly improves corrosion resistance to atmospheric environments but does not have much effect against liquid corrosives. The alloying elements produce a tight, dense adherent rust film, but in acid or alkaline solutions corrosion is about equivalent to that of carbon steel. However, the greater strength permits thinner walls in process equipment made from low-alloy steel. 

Tubercles are mounds of corrosion product and deposit that cap localized regions of metal loss. Tubercles can choke pipes, leading to diminished flow and increased pumping costs (Fig. 3.1). Tubercles form on steel and cast iron when surfaces are exposed to oxygenated waters. Soft waters with high bicarbonate alkalinity stimulate tubercle formation, as do high concentrations of sulfate, chloride, and other aggressive anions. 

Pitting is also promoted by low pH. Thus, acidic deposits contribute to attack on stainless steels. Amphoteric alloys such as aluminum are harmed by both acidic and alkaline deposits (Fig. 4.4). Other passive metals (those that form protective corrosion product layers spontaneously) are similarly affected. 

Corrosion of industrial alloys in alkaline waters is not as common or as severe as attack associated with acidic conditions. Caustic solutions produce little corrosion on steel, stainless steel, cast iron, nickel, and nickel alloys under most cooling water conditions. Ammonia produces wastage and cracking mainly on copper and copper alloys. Most other alloys are not attacked at cooling water temperatures. This is at least in part explained by inherent alloy corrosion behavior and the interaction of specific ions on the metal surface. Further, many dissolved minerals have normal pH solubility and thus deposit at faster rates when pH increases. Precipitated minerals such as phosphates, carbonates, and silicates, for example, tend to reduce corrosion on many alloys. 

The anodically produced acid is neutralized by the alkaline mortar (CaO). Corrosion is then possible only if the supply of alkali at the steel surface is consumed and the steel becomes active. This process is possible only under certain circumstances after a very long incubation period. Apparently in steel-concrete foundations the possible current densities are so small that this case never arises. The possibility of danger has to be verified with thin outer coatings where deliming has been noticed on the steel surface. 

Cathodic protection of reinforcing steel with impressed current is a relatively new protection method. It was used experimentally at the end of the 1950s [21,22] for renovating steel-reinforced concrete structures damaged by corrosion, but not pursued further because of a lack of suitable anode materials so that driving voltages of 15 to 200 V had to be applied. Also, from previous experience [23-26], loss of adhesion between the steel and concrete due to cathodic alkalinity [see Eqs. (2-17) and (2-19)] was feared, which discouraged further technical development. 

The passivating action of an aqueous solution within porous concrete can be changed by various factors (see Section 5.3.2). The passive film can be destroyed by penetration of chloride ions to the reinforcing steel if a critical concentration of ions is reached. In damp concrete, local corrosion can occur even in the presence of the alkaline water absorbed in the porous concrete (see Section 2.3.2). The Cl content is limited to 0.4% of the cement mass in steel-concrete structures [6] and to 0.2% in prestressed concrete structures [7]. 

The information in Sections 2.2, 2.4 and 3.3 is relevant for protection criteria. Investigations [43] with steel-concrete test bodies have shown that even in unfavorable conditions with aerated large-area cathodes and small-area damp anodes in Cl -rich alkaline environments, or in decalcified (neutral) surroundings with additions of CU at test potentials of (/f.y.cuso4 = -0.75 and -0.85 V, cell formation is suppressed. After the experiments had proceeded for 6 months, the demounted specimens showed no recognizable corrosive attack. 

Ignoring alkaline service. Just because strong alkalies do not cause severe overall corrosion in carbon steel or stainless steel, don t overlook stress coito-sion, cracking, or effects on other materials. 

Alkalinity Bicarbonate (HCO3), carbonate (CO3), and hydrate (OH), expressed as CaCOj. Causes foaming and carryover of solids with steam. Can cause embrittlement of boiler steel. Biocarbonate and carbonate generate COj in steam, a source of corrosion. 

Caustic Embrittlement—a form of stress corrosion cracking that occurs in steel exposed to alkaline solutions. 

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