Corrosion process precipitation

Foulants enter a cooling system with makeup water, airborne contamination, process leaks, and corrosion. Most potential foulants enter with makeup water as particulate matter, such as clay, sdt, and iron oxides. Insoluble aluminum and iron hydroxides enter a system from makeup water pretreatment operations. Some well waters contain high levels of soluble ferrous iron that is later oxidized to ferric iron by dissolved oxygen in the recirculating cooling water. Because it is insoluble, the ferric iron precipitates. The steel corrosion process is also a source of ferrous iron and, consequendy, contributes to fouling. 

As the film dissolves more oxide film is formed, i.e. the metal/oxide interface progresses into the metal, and the overall rate may be low enough to be acceptable for a particular process. In other cases, the corrosion products precipitate on the surface of the oxide and either accelerate the overall rate by enhancing diffusion of ions through the porous outer layers or, when less porous layers are formed, access of hydrogen ions to the inner oxide surface is reduced thus decreasing the rate. 

The following model of the corrosion process can be proposed based on the wealth of data provided by the combined application of SPFM, contact AFM, and IRAS At low RFl, the principal corrosion prodnct, hydrated alnminnm snlfate, is solid. It acts as a diffn-sion barrier between the acid and the alnminnm snbstrate and prevents fnrther corrosion. The phase separation observed between the acid and the salt at low RH strongly snggests that the salt inhibits fnrther corrosion once it precipitates. At high RH, on the other hand, alnminnm snlfate forms a liqnid solntion. Snlfnric acid mixes with this solntion and reaches the nnderlying snbstrate, where fnrther reaction can occnr. The flnid snlfate solntion also wets the snrface better and thns spreads the snlfnric acid. The two processes assist each other, and the corrosion proceeds rapidly once the critical RH of 80-90% is reached. 

Absorbent availability could have significant impact upon process costs. Most spent alkali streams could be used depending on the other impurities present. Corrosion or precipitation tests should be considered for these cases. Any other readily available alkali, like sodium carbonate or sodium hychloride, should be considered. Slurry solutions of lime or limestone should be avoided because of past operating problems. 

It is therefore believed that at pH 6 and greater the corrosion process is localised and large local concentrations of ferrous iron are achieved. At pH 6 the oxidation to ferric iron is very rapid ( ) and precipitation of Fe(0H)j occurs to exhibit localised corrosion or "flash-rust" spots. At pH 5 and below a small but finite uniform dissolution of the iron substrate occurs. However, in this pH range the oxidation of the ferrous dissolution product to ferric ion is considerably slower, by almost 1000 times, and hence "flash rusting" is not observed. 

Under indoor conditions, in the same way than outdoors, it is necessary the presence of surface humidity for corrosion to occur due to the electrochemical nature of the atmospheric corrosion process however, in indoor conditions there are no precipitations and the presence of surface water depends mainly on water content in the air and changes in temperature on the surface, as well as the presence of hygroscopic substances on the metallic surface. 

The atmospheric corrosion rate of metals depends mainly on TOW and pollutants however, if the differences in the corrosion process between outdoor and indoor conditions are taken into account, the influence of direct precipitation such as rain is very important for outdoor and negligible for indoor conditions. The acceleration effect of pollutants could change depending on wetness conditions of the surface, so the influence of the rain time and quantity should be very important in determining changes in corrosion rate. 

Chloride ions move through the corrosion products layer and arrive at metal surfaces producing a notable acceleration in corrosion rate. A catalyser role has been suggested for anions in the corrosion process. Chloride ions are very soluble, that is why they are easily removed from metallic surfaces due to liquid precipitation. Usually, a low concentration of chloride ions is determined in corrosion products on outdoor exposures. A relation between chloride concentration present in corrosion products and corrosion rate has been reported. 

The corrosion process can be inhibited by the addition of phosphate or polyphosphate ions [344], inorganic inhibitors as, for example, chromate ions [336], adsorbed alcohols [345], adsorbed amines, competing with anions for adsorption sites [339,] as well as saturated linear aliphatic mono-carboxylate anions, CH3(CH2)n-2COO , n = 7 — 11, [24]. In the latter case, the formation of the passive layer requires Pb oxidation to Pb + by dissolved oxygen and then precipitation of hardly soluble lead carboxylate on the metal surface. The corrosion protection can also be related to the hydrophobic character of carboxylate anions, which reduce the wetting of the metal surface. 

Typically a limit of, say, 2.0 ppm total iron is set for the maximum permitted in a system, but this usually has more to do with the tolerance of ongoing corrosion processes than with an effort to prevent problems occurring in the first place due to the presence of incoming iron. Where iron is present in cooling water, not only can it form a precipitated sludge and foul the system but also it can cause both electrochemical and biologically induced corrosion processes to occur. 

A way to further minimize corrosion is by adding base to the feed or reactor, so dial acids formed during the oxidation reaction are immediately neutralized. However, one must then deal with the resulting salts. Whether formed during reaction or already contained in the feed, salts will quickly precipitate in supercritical water. As these salts tend to adhere to and accumulate on the reactor walls and other surfaces within the reactor, they can inhibit and ultimately block process flow unless they are removed or their accumulation is controlled. Nonsalt solids (e.g., metal oxides, grit), by contrast, have little tendency to stick to process surfaces but can be a problem with respect to erosion and system pressure control. Methods that have been developed to manage and/or minimize the impact of corrosion, salt precipitation/accumulation, and solids handling are discussed in Sections 6.5 and 6.6. 

Electrocatalysis in metallic corrosion may be classified into two groups Adsorption-induced catalyses and solid precipitate catalyses on the metal surface. In general, the bare surface of metals is soft acid in the Lewis acid-base concept and tends to adsorb ions and molecules of soft base forming the covalent binding between the metal surface and the adsorbates. The Lewis acidity of the metal surface however may turn gradually to be hard as the electrode potential is made positive, and the bare metal surface will then adsorb species of hard base such as water molecules and hydroxide ions in aqueous solution. Ions and molecules thus adsorbed on the metal surface catalyze or inhibit the corrosion processes. Solid precipitates, on the other hand, are produced by the combination of hydrated cations of hard acid and anions of hard base forming the ionic bonding between the cations and the anions on the metal surface. 

Many of the effects of internal oxidation, both on the overall corrosion process for an alloy and on influencing the mechanical, magnetic, etc., properties of the alloy, are intimately related to the morphology of the oxide precipitates. The following is a rather qualitative discussion of the factors, which influence the size, shape, and distribution of internal oxides. 

The local current density within corrosion pits may be extremely large. If the precipitation of corrosion products does not occur, the metal dissolution is controlled by charge transfer and ohmic effects, and hence the corrosion process is potential dependent. This situation requires a sufficiently acidic solution to avoid the precipitation of insoluble oxides or a still not saturated or supersaturated pit electrolyte with no formation of a salt layer. Pitting at potentials close to the critical value Ep occurs usually with moderately small local current densities / c,p that, however, may increase with potential to extremely... 

The resulting corrosion processes not only lead to leakage but also may induce failure of system components owing to clogging with corrosion products (e.g. scales and precipitates). Corrosion inhibitors for water treatment should be effective at variable water composition, temperature, and flow conditions for a wide range of inhibitor concentrations. They should protect all exposed metals and should not be aggressive to other materials in the system (e.g. solder, rubber, and plastics). Furthermore, they should not stimulate the buildup of scales, thermally isolating... 

The mechanism is that deposits of corrosion products, or salts precipitated because of the corrosion process, are worn off, dissolved or prevented from being formed, so fliat the material surface becomes metallically clean and therefore more active. In extreme cases, erosion corrosion may be accompanied by pure mechanical erosion, by which sohd particles in flie fluid may tear out particles from the material itself and cause plastic deformation, which may make the metal even more active. 

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