Corrosion concentration effects

Economizer corrosion rates are enhanced by higher heat-transfer rates excessive heat flux may create localized nucleate boiling zones where gouging, as a result of chemical concentration effects, can occur. Air heaters are also located in the exit gas system. They do a job similar to that of economizers except that they preheat combustion air. 

Other forms of concentration-cell corrosion include Caustic gouging Saline corrosion Combination of free caustic and concentrating effect causes severe metal wastage. High chlorides and sulfates, result in corrosion from depolarization and depassivation effects... 

Low pH localized corrosion Requires an acid source and a concentration effect... 

Corrosion may be especially rapid under existing deposits where amine is absent and the pH level is lower than that of the bulk water, as incongruent phosphate effects and concentrating effects further contribute to the problem. 

Acids, when used as scale inhibitors, are extremely corrosive. Their effectiveness has been laboratory tested. Parameters include acid type, metallurgy, temperature, inhibitor type and concentration, duration of acid-metal contact, and the effect of other chemical additives [279]. Lead and zinc sulphide scale deposits can be removed by an acid treatment [922]. 

Corrosion is effectively controlled by using chemical inhibitor systems which are added to the glycol to make a complete coolant concentrate formulation commonly referred to as permanent engine coolant. 

Crude 10% sodium hydroxide containing sodium chloride is purified in a similar manner to the product of the causticization process. The water is evaporated in nickel or nickel-clad steel (to reduce corrosion) multiple-effect evaporators to about 50% sodium hydroxide concentration. At this concentration, sodium chloride is only about 1% soluble (2%, on a dry basis) in the more concentrated caustic so that the bulk of it crystallizes out and is filtered off. This quite pure sodium chloride is recycled to the cells. Lor many purposes, such as for pulp and paper production, this purity of 50% sodium hydroxide is quite acceptable. If higher purities are required, sodium hydroxide may be separated from residual water and salt by chilling to the double hydrate crystals NaOH 2HiO, m.p. about 6°C, or as NaOH 3.5HiO, with a m.p. of about 3°C, or by counter-current extraction [9]. The sodium hydroxide obtained after these steps contains 2-3 ppm sodium chloride, equivalent to the purity of the mercury cell product ( rayon grade ) [10]. Concentrations of 73% and 100% sodium hydroxide (see details, Section 7.5) are also marketed. 

A chemical substance which, when added to the environment, usually in small concentrations effectively decreases corrosion, after concentrations. 

Two subtle corrosion effects can occur when a single metal is in contact with an electrolyte -differential aeration and crevice corrosion. Differential aeration can cause corrosion when no obvious galvanic cells are in evidence. To illustrate this effect, suppose we have a cell with a copper anode and cathode. If the concentration of the electrolyte and the temperature of each cell compartment is the same, no potential is generated and no corrosion occurs. However, bubble O2 into the one compartment, which becomes the cathode compartment, and corrosion will occur in the other, which forms the anode compartment. Differential aeration is, in fact, a concentration effect, and can be understood by using the Nernst equation. Electrons will flow from anode to cathode and the anode will corrode. 

Interruptions in the stray current. Stray currents produced by rail traction systems are non-stationary, and thus the effect of interruptions of the current should be taken into consideration. In fact, gradients of ionic concentration in the pore solution near the steel surface, produced by the depletion of alkalinity due to the anodic reaction and increase in chloride concentration due to migration, can be mitigated during the interruption of current. Therefore, interruptions in the stray current may have a beneficial effect, as shown in Figure 9.6 where results of tests with continuous appHcation of the current are compared with tests with circulation of current at alternated hours (lon-loff). The periodical interruption of current had a beneficial effect, since it increased the charge required for initiation of corrosion. This effect was remarkable in cement pastes with chloride contents lower than 0.4 % by mass of cement... 

The electrode pH and potential at an active crack tip surface may be significantly different from those on boldly exposed surfaces of a material. Low pH conditions can lead to local dissolution of the metal and crack tip blunting that reduces the stress concentration effects. In contrast, low pH conditions favor hydrogen generation and consequently increase risk of HE corrosion. The reduction in ductility associated with HE corrosion may produce sharp crack tips, which in turn may increase stress concentration effects for any synergistic SCC or CF (4). 

Tung et al. (2001) have studied two distinct scavengers, but the identities of those scavengers were not revealed in their research. They used brine as the medium for the corrosion tests. The sulfide concentrations put to test were 100,200, and 300 ppm, and the scavenger/sulfide ratios were 1/1,2/1, and 4/1. These concentrations were tested at pHs of 5 and 7 and at ambient temperature. They finally concluded that the corrosion inhibition effects of the two chemical additives can be as high as over 95%. 

Calcinm nitrite is the principal corrosion inhibitor available to stop corrosion that is compatible with concrete in the casting process. As stated in Section 10.2, it is accepted by FHWA as an alternative to FBECR for protection against chloride induced corrosion. FHWA research shows that if sufficient nitrite is added to the concrete mix to ensure a chloride to nitrite ratio of less than 1.0 at rebar depth, then the nitrite will prevent corrosion. Obviously this is feasible in marine conditions where the chloride level is known (and assuming no concentration effects) but may be more difficult in other situations. 

Stainless steels are resistant to corrosion by most salts. The exceptions are the halide salts that cause pitting, crevice corrosion, and SCC. Of these salts, those containing chlorides are the most corrosive, followed by fluoride, bromide, and iodide salts. Stainless steels with higher chromium, molybdenum, and nitrogen concentrations will resist pitting and crevice corrosion more effectively. Austenitic stainless steels with higher molybdenum and nickel, ferritic stainless steels with no nickel or copper, and duplex stainless steels will resist SCC. 

Another important factor is the highly corrosive environment produced by the accumulation of dirt, vegetation, and other debris in metallic entrapment areas [7]. These areas remain wet almost continuously due to the presence of this poultice and produce a stationary electrolyte environment. The result is a concentrating effect of salts and acids similar to the phenomena observed in crevices. When steels are exposed to this poultice environment, the corrosion process becomes autocatalytic leading to rapid deterioration. 

Figure 9-7 shows the iron concentration of test solutions as a function of the HEDP concentration at the end of gravimetric measurements (24 h) (Kalman et al., 1994). The iron content was determined separately in dissolved and precipitated (rust) form. The variation of total iron content is in accordance with the corrosion rate of carbon steel, showing a minimum curve. At lower HEDP concentrations, high amounts of iron exist in the form of rust, indicating a corrosion protection effect of the oxide layer on the carbon steel surface. At higher concentrations (c> 10 M), HEDP keeps the total amount of iron in the solution phase. These results indicate that complex formation between HEDP and iron ions has an important effect on the inhibition effect of HEDP. The... 

Zn2 A with increasing zinc ion concentration in the solution. The high corrosion inhibition effect of the Zn /HEDP mixture (t/ix >95% at Zn /HEDP=2-4 molar ratio) is due to precipitation of these polymeric complexes with charge, which enables electrostatic binding to be established with the metal surface, thus forming a protective layer. The... 

---