Active corrosion

In the following, the most typical modes of corrosion—other than the above discussed unifonn dissolution (active corrosion) and localized pitting and crevice corrosion (local active dissolution)—are briefly presented. 

Oxygen is a prime factor in the corrosion of system materials and the release, activation, and redeposition of activated corrosion products. 

Planktonic counts (in water samples) are usually unreliable as an indicator of active corrosion. The presence of any sulfate reducers in the water, however, indicates much higher concentrations of these organisms on surfaces somewhere in the system. 

Stainless steels tend to pit in acid solutions. Pits form local areas of metal loss associated with breakdown of a protective oxide layer. Breakdown is stimulated by low pH as well as by the decrease of dissolved oxygen in occluded regions. Small, active pit sites form and remain stable because of the large ratio of cathodic surface area (unattacked metal surface) to the pit area. Active corrosion in the pit cathodically protects immediately adjacent areas. If conditions become very severe, pitting will give way to general attack as more and more of the surface becomes actively involved in corrosion. 

Pits form grooves or furrows on wall surfaces on stainless steels (Figs. 7.5 and 7.6). Contents leak out of pit interiors and depassivate areas immediately below the active corrosion sites. Grooves form, fol-... 

Correspondingly, amphoteric aluminum materials suffer active corrosion by the anodic partial reaction ... 

Passivating inhibitors act in two ways. First they can reduce the passivating current density by encouraging passive film formation, and second they raise the cathodic partial current density by their reduction. Inhibitors can have either both or only one of these properties. Passivating inhibitors belong to the group of so-called dangerous inhibitors because with incomplete inhibition, severe local active corrosion occurs. In this case, passivated cathodic surfaces are close to noninhibited anodic surfaces. 

A particular problem arises in the anodic protection of the gas space because the anodic protection does not act here and there is a danger of active corrosion. Thus these endangered areas have to be taken account of in the design of chemical... 

If the amount of metal removal by erosion is significant the surface will probably be continually active. Metal loss will be the additive effect of erosion and active corrosion. Sometimes the erosion rate is higher than that of active corrosion. The material selection judgment can then disregard coirosion and proceed on the basis of erosion resistance provided the corrosion rates of aetive surfaces of the alloys considered are not much different. As an example of magnitudes, a good high-chromium iron may lose metal from erosion only a tenth as fast as do the usual stainless steels. 

Primers are required to be resistant to all of the same fluids and environments as the adhesive, and are in addition expected to be compatible with secondary finishes such as corrosion and fluid resistant primers applied to cured bond assemblies. The most commonly used primers for 250°F cured epoxy adhesives also have active corrosion inhibitors themselves to combat corrosion at bondlines. This last requirement is somewhat dated, evolving from the severe corrosion and delamination problems experienced before U.S. airframe manufacturers adopted durable surface treatments. 

Feed solutions are usually made up at a water to chemical ratio of 2 1 to 8 1 (on a weight basis) with the usual ratio being 4 1 with a 20-minute detention time. Care must be taken not to dilute ferric sulfate solutions to less than 1 percent to prevent hydrolysis and deposition of ferric hydroxide. Ferric sulfate is actively corrosive in solution, and dissolving and transporting equipment should be fabricated of type 316 stainless steel, rubber, plastics, ceramics, or lead. 

The basic mechanism of passivation is easy to understand. When the metal atoms of a fresh metal surface are oxidised (under a suitable driving force) two alternative processes occur. They may enter the solution phase as solvated metal ions, passing across the electrical double layer, or they may remain on the surface to form a new solid phase, the passivating film. The former case is active corrosion, with metal ions passing freely into solution via adsorbed intermediates. In many real corrosion cases, the metal ions, despite dissolving, are in fact not very soluble, or are not transported away from the vicinity of the surface very quickly, and may consequently still... 

Similar initial reactions occur on many metals such as iron and cobalt. This intermediate can now react further in one of two ways. Oxidation and protonation of the intermediate to Ni(II) leads to dissolved nickel ions (active corrosion) which are unable to passivate the metal ... 

For some non-ferrous metals (copper, lead, nickel) the attack by sulphuric acid is probably direct with the formation of sulphates. Lead sulphate is barely soluble and gives good protection. Nickel and copper sulphates are deliquescent but are gradually converted (if not leached away) into insoluble basic sulphates, e.g. Cu Cu(OH)2)3SO4, and the metals are thus protected after a period of active corrosion. For zinc and cadmium the sulphur acids probably act by dissolution of the protective basic carbonate film. This reforms, consuming metal in the process, redissolves, and so on. Zinc and cadmium sulphates are formed in polluted winter conditions whereas in the purer atmospheres of the summer the corrosion products include considerable amounts of oxide and basic carbonate. ... 

To trigger off an anodic protection system for stainless-steel coolers cooling hot concentrated sulphuric acid when the potential moves towards that of active corrosion. 

To prompt remedial action when stainless-steel agitators in a phosphoric-acid-plant reactor show a potential shift towards a value associated with active corrosion due to an increase in corrosive impurities in the phosphate rock. 

The electrochemical examination of fusion joints between nine pairs of dissimilar metal couples in seawater showed that in most cases the HAZ was anodic to the weld metals" . Prasad Rao and Prasanna Kumarundertook electrochemical studies of austenitic stainless steel claddings to find that heat input and 5Fe content significantly affected the anodic polarisation behaviour under active corrosion conditions whilst Herbsleb and Stoffelo found that two-phased weld claddings of the 24Cr-13Ni type were susceptible to inter-granular attack (IGA) as a result of sensitisation after heat treatment at 600°C /pa was unaffected by heat input. 

Corrosion or mixed potentials (a) Active corrosion in acid solutions (b) Passive metal in acid solutions Potential dependent on the redox potential of the solution and the kinetics of the anodic and cathodic reactions. Potential dependent on the kinetics of the h.e.r. on the bare metal surface. Potential is that of an oxide-hlmed metal, and is dependent on the redox potential of the solution. Zn in HCI Stainless steel in oxygenated H2SO4... 

The development of high radiation fields surrounding areas of active corrosion... 

Recently the patina formed in the atmosphere on the roof of the Stockholm City Hall was analyzed. Several components of the patina were identified, including brochantite (CuSC>4- 3Cu(OH)2), antlerite (Cu3(OH)4-SC>4), and basic cupric carbonate (Cu2C03(0H)ftH20). At the Rodin Museum in Philadelphia, active corrosion of Rodin s The Thinker was ascribed in 1992 to primarily brochan-... 

Anomalous cases were noted in which this generalization did not hold. These very empirical measurements were followed by more thorough studies (IS). Thin paint films with very low electrical resistance show active corrosion potentials which become more positive as the paint film was increased in thickness. Shapes of the potential/time curves were misleading as a guide to ultimate coating protective properties. 

From a theoretical point of view, the gel layer is a barrier that reduces further hydrolysis of the silicate network, and is supposed to be more stable than the glass matrix, thus reducing the overall rate of corrosion. However, gel exfoliation may momentarily re-activate corrosion, at least locally. No clear trend was observed for the presence of the crystalline secondary phases identified at the surface of the corroded HT samples. The most abundant minerals are aluminosilicates, calcium phosphates, Fe- and Mg-rich minerals, and zeolites their role in the scavenging or release of metals remains ambiguous, although many mineral phases identified bear traces of metals. 

Figure 16.10 Active corrosion of titanium by aqueous acid.

Figure 16.12 Titanium with metastable passivity. If Eh or io are not quite high enough, the metal may go from passive to active behavior without warning. If this happens, the corrosion current will be high (cf. normal active corrosion, Fig. 16.10).

Pseudomonas sp. Facultatively aerobic, very common heavy slime producer. Can also initiate active corrosion by consuming oxygen and initiating differential oxygen concentration cells. 

The initial surface composition of boiler tubing, prior to its installation will have an important impact on the amount and type of activated corrosion products in an aqueous reactor coolant. Consequently, the type of thermal pre-treatment the tubing undergoes, for example, for mechanical stress release,will affect the surface oxide film, and ultimately, the corrosion behavior. This particular work has been directed toward characterization of surface oxide films which form on Inconel 600 (nominal composition 77% Ni, 16% Cr, 7% Fe, — a tradename of Inco Metals Ltd., Toronto Canada) and Incoloy 800 (nominal composition 31% Ni, 19% Cr, 48% Fe 2% other, — a tradename of Inco Metals Ltd., Toronto, Canada) heated to temperatures of 500-600°C for periods of up to 1 minute in flowing argon. These are conditions equivalent to those experi enced by CANDU(CANadian Deuterium Uranium)ractor boiler hairpins during in situ stress relief. 

When this conjugation occurs, the level of active (corrosive) acid is substantially decreased. No simple quantitive correlation has been shown between the acidity (pKa) of acids in hydrocarbon formulation and low polar solvents (Coetzee, 1967). Acid-base interaction with and without proton transfer (PT) (BH+A B...(HA)m) has been related to acid and base enthalpies of reaction (Pawlak and Bates, 1982), the infrared carbonyl stretching band and gradual appearance of the asymmetric COO band (Lindeman and Zundel, 1972 Magonski and Pawlak, 1982), changes in pH (Kuna et al., 1982 Pawlak et al., 1982), NMR proton chemical shifts (Magonski and Pawlak, 1982), and dipole moments (Sobczyk and Pawelka, 1979). These parameters depend upon the acid-base strength of the partners, ApKa(PT) the difference between the pKa(acceptor) and pKa(donor) on the water scale (Sobczyk, 2001). 

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