Corrosion inhibition aqueous systems

The second approach, changing the environment, is a widely used, practical method of preventing corrosion. In aqueous systems, there are three ways to effect a change in environment to inhibit corrosion (/) form a protective film of calcium carbonate on the metal surface using the natural calcium and alkalinity in the water, (2) remove the corrosive oxygen from the water, either by mechanical or chemical deaeration, and (3) add corrosion inhibitors. 

Azole compounds, poly-N-vinylimidazole (PVI-1) and 2-undecylimidazole (UDI), are studied as alternative inhibitors to benzotriazole (BTA) for copper corrosion in aqueous systems using electrochemical techniques. It is shown that UDI, either as a cast film or dissolved in solution at concentrations as low as 7 X inhibits oxygen reduction on... 

Anodic passivation of steel surfaces can be efficiently achieved by metal chromates. Chromates of Intermediate solubility (e.g., zinc chromate and strontium chromate) allow a compromise between mobility in the film and leaching from the film to be achieved. Chromates inhibit corrosion in aqueous systems by formation of a passivating oxide film. The effectiveness of chromate inhibitors in aqueous systems depends on the concentration of other ionic species in solution, for example, chloride. Synthetic resin composition can also significantly influence the effectiveness of chromate pigments. The effect appears to be related to the polarity of the resin (20) chromate pigments appear to be less effective in resins of low polarity. 

Silicates. For many years, siUcates have been used to inhibit aqueous corrosion, particularly in potable water systems. Probably due to the complexity of siUcate chemistry, their mechanism of inhibition has not yet been firmly estabUshed. They are nonoxidizing and require oxygen to inhibit corrosion, so they are not passivators in the classical sense. Yet they do not form visible precipitates on the metal surface. They appear to inhibit by an adsorption mechanism. It is thought that siUca and iron corrosion products interact. However, recent work indicates that this interaction may not be necessary. SiUcates are slow-acting inhibitors in some cases, 2 or 3 weeks may be required to estabUsh protection fully. It is beheved that the polysiUcate ions or coUoidal siUca are the active species and these are formed slowly from monosilicic acid, which is the predorninant species in water at the pH levels maintained in cooling systems. 

R. P. Kreh. Method of inhibiting corrosion and scale formation in aqueous systems. Patent US 5073339, 1991. 

B. H. Cartledge Oak Ridge National Lab.) While I appreciate Huddle and Anderson s (Lecture 41) sympathetic reference to any work on inhibition by the pertechnetate ion, I must record any objection the interpretation of any views which they present. The processes of corrosion and inhibition are usually too complex to be related uniquely to single anodic and cathodic reactions. Thus, in aerated aqueous systems containing one of the inhibitors of the X04 type, there are at least three possible cathodic processes namely, reduction of oxygen, of hydrogen ions, or of the inhibitor anion, if it is reducible. The kinetics, as well as the thermodynamics, of the several reactions, determines which of them will predominate in any particular case. 

Aqueous systems are by far the most common corrosive environments to which corrosion inhibitors are applied. Water is a powerful solvent capable of carrying many different ions at the same time, so requirements for corrosion inhibition may vary greatly, depending on the type and amount of dissolved species present. Because there is no universal inhibitor for water systems, an inhibitor that may be satisfactory for one system may be ineffective or even harmful in another. The main factors that may be considered in the application of corrosion inhibitors to aqueous systems are salt concentration, pH, dissolved oxygen concentration, and the concentration of interfering species (Liu and Gao 2010). 

A 100% active, liquid quaternary compound offering excellent antistatic, lubricating, fiber softening and corrosion inhibiting properties in aqueous and non-aqueous systems. 

In Figure 15.15, corrosion inhibition efficiency data are particularly shown for systems containing the surfactant ARIS in structurally distinct media. The metal surface affected was API5LX Gr X42 steel. The surfactant was previously dissolved in NaCl aqueous solutions, at 0.5 M or 1.0 M salt concentrations. The tests were carried out either using micellar or microemulsion systems, at 30°C and 60°C. The microemulsion system was prepared with the surfactant NaCl aqueous solutions, butan-l-ol as cosurfactant CIS ratio = 1.0) and kerosene as oil phase. 

It is important to understand the mechanism of corrosion inhibition promoted by surfactant-based systems. The transition of the metal-solution interface from an active dissolution state to a passivation state is highly important in petroleum fields. Normally, surfactants are added to aqueous media to occupy the interface, hence reducing corrosion of the pipelines. It is known that increasing surfactant concentrations reduce interfacial tensions, as a result of enhanced aggregation and physical adsorption upon micelle formation at concentrations above the CMC. 

FIGURE 15.15 Efficiency of corrosion inhibition on API5LX GrX42 steel with surfactant ARIS as inhibitor at 30°C and 60°C, dissolved in micellar or microemulsion systems. The surfactant had been previously dissolved in (a) 0.5 M NaCl aqueous solution or... 

Research at Monash University has targeted an extensive range of rare earth carboxylates of low toxicity to investigate and identify successful corrosion inhibition systems, and to determine the influence of stracture on inhibition efficiency for mild steel and aluminiiun in aqueous chloride solutions. ... 

Removing suspended solids, decreasing cycles of concentration, and clarification all may be beneficial in reducing deposits. Biodispersants and biocides should be used in biofouled systems. Simple pH adjustment may lessen precipitation of certain chemical species. The judicious use of chemical corrosion inhibitors has reduced virtually all forms of aqueous corrosion, including underdeposit corrosion. Of course, the cleaner the metal surface, the more effective most chemical inhibition will be. Process leaks must be identified and eliminated. 

Environments are either gases or liquids, and inhibition of the former is discussed in Section 17.1. In some situations it would appear that corrosion is due to the presence of a solid phase, e.g. when a metal is in contact with concrete, coal slurries, etc. but in fact the corrosive agent is the liquid phase that is always present. Inhibition of liquid systems is largely concerned with water and aqueous solutions, but this is not always so since inhibitors may be added to other liquids to prevent or reduce their corrosive effects — although even in these situations corrosion is often due to the presence of small quantities of an aggressive aqueous phase, e.g. in lubricating oils and hydraulic fluids (see Section 2.11). 

Nature of the environment This is usually water, an aqueous solution or a two- (or more) component system in which water is one component. Inhibitors are, however, sometimes required for non-aqueous liquid systems. These include pure organic liquids (Al in chlorinated hydrocarbons) various oils and greases and liquid metals (Mg, Zr and Ti have been added to liquid Bi to prevent mild steel corrosion by the latter ). An unusual case of inhibition is the addition of NO to N2O4 to prevent the stress-corrosion cracking of Ti-6A1-4V fuel tanks when the N2O4 is pressurised... 

The use of various substances as additives to process streams to inhibit corrosion has found widespread use and is generally most economically attractive in recirculation systems, however, it has also been found to be attractive in some once-through systems such as those encountered in the petroleum industry. Typical inhibitors used to prevent corrosion of iron or steel in aqueous solutions are chromates, phosphates, and silicates. In acid solutions, organic sulphides and amides are effective. 

Inhibitors are very important for corrosion of irons and other metals. The corrosion of irons and other metals in aqueous solutions can frequently be minimized or inhibited by the addition of soluble chromates, molybdates, silicates, and amines or other chemicals, singly or in combination. Such materials are called inhibitors and are generally attractive for use in recirculating systems or closed systems. They are also used in neutral or very slightly acid solutions. The concentration of an inhibitor for maximum control depends on the solution, composition, temperature, velocity, metal system, and the presence of dissimilar metals in contact in the solution. Care should be taken in the selection and application of inhibitors, since in some instances they can increase localized attack. 

Since molten salts are very reactive, due especially to the appearance of temperature in the exponential part of rate equations, it is unlikely that corrosion will be kinetically inhibited if thermodynamically predicted, as is often encountered in aqueous solutions. Moreover, when assessing corrosion possibilities, the whole system including container materials needs to be considered, since the latter are rarely inert in contact with melts. 

Flash rusting is the phenomenon of formation of small brown rust spots that appear very soon after application of waterbased systems directly to steel. The problem is more prominent when the steel surface is active (for example, after blasting) and the pH is <7. To combat this problem, flash-rust inhibitors are added to waterbased coatings. They should have good water solubility. Flat surfaces are relatively easy to protect, while weld areas are considered difficult. Flash-rust inhibition at a weld area can be achieved by a combination of different products. Flash-rust inhibitors are typically based on ammonium and sodium nitrites, benzoates, metaborates and phosphates, occasionally in combination with materials such as morpholine and amino methyl propanol. Some other organic salts are also used, such as amine salts of 2-mercaptobenzothiazolyl succinic acid and calcium or barium salts of linear alkyl naphthalene sulfonic acids. Typically they are used at levels of 0.1 to 1.0 % of the total mass of paint. They also help in preventing in-can corrosion upon storage of aqueous paint. 

Published literature also deals with the effect of the incorporation of REM salts in mixed aqueous media in order to inhibit the corrosion of different metallic materials. An example is the addition of Ce ions to flowing ethylene glycol-water solutions to inhibit the corrosion of aliuninum alloy 3003 used in refrigerating systems (Liu and Cheng, 2011). However, in recent years, the most innovative developments in this area of REM salt corrosion inhibitors have involved the study of the synergic effects of the mixtures of these rare earths with various organic and inorganic compounds. 

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