Ferric ion corrosion

Silica and silicate deposits cannot be removed with HCI. Where such scales are a problem, hydrofluoric acid is used either alone or in conjunction with HCI (or other acids). In the latter case, the HF is usually generated in situ by means of the addition of NH4HF2. The HCI/HF mixture has the added features of being more aggressive toward iron scales and reducing base metal corrosion by Fe in magnetite and hematite deposits, a result attributable to the formation of a strong complex between HF and Fe3+.97 pqc additional discussion, see the section on ferric ion corrosion. 

Most organic corrosion inhibitors used in cleaning solutions will not prevent ferric ion corrosion. Special chemicals were developed to reduce this problem. Alfandry demonstrated that hexamethylenetetramine will protect steel in HCI, but the protection is reduced by the presence of Fe + ions. However, the protection provided to steel in sulfuric acid is improved by the addition of ferric ions to a solution inhibited by phenylthiourea. Streicher reported that ferric ion corrosion of steel in sulfuric acid can be reduced by a mixture of diorthotolylthiourea and an anionic surfactant, such as an alkyl-aryl sodium sulfonate. Sulfonium salts also were used as ferric ion inhibitors. These inhibitors are effective in a variety of mineral and organic acids, but they are most effective in acidic (pH of 5) EDTA solutions. The sulfonium salts were shown to electroreduce at the corrosion potential to form a hydrophobic product (hydrocarbon) that may enhance the protection of the steel from oxidation by the ferric ion. 

Historically, ferrous sulfamate, Fe(NH2S02)2, was added to the HNO scmbbing solution in sufficient excess to ensure the destmction of nitrite ions and the resulting reduction of the Pu to the less extractable Pu . However, the sulfate ion is undesirable because sulfate complexes with the plutonium to compHcate the subsequent plutonium purification step, adds to corrosion problems, and as SO2 is an off-gas pollutant during any subsequent high temperature waste solidification operations. The associated ferric ion contributes significantly to the solidified waste volume. 

Zirconium is totally resistant to attack of hydrochloric acid in all concentrations to temperatures well above boiling (Fig. 2). Aeration has no effect, but oxidizing agents such as cupric or ferric ions may cause pitting. Zirconium also has excellent corrosion resistance to hydrobromic and hydriodic acid. 

The ease with which the ferrous ion can be oxidized to a ferric ion in the electrowinning cell furthers this reaction. Attack on the copper is most apparent at the solution line, where it results in corrosion of the loops supporting the cathodes, leading to dropped cathodes. 

During acid cleaning or severe acid upsets, ferric-ion concentration may increase, albeit much more slowly than ferrous-ion concentration, to high levels. Resulting corrosion can be severe. Iron is oxidized, and... 

Hence, copper heat exchanger tubes handling acetic acid can he more seriously corroded at low temperatures than at high temperatures. Sulfuric acid at room temperature is handled routinely in carbon steel drums and tanks when water concentration is low, but it becomes extremely corrosive as water concentration increases. As ferric-ion concentration increases during acid cleaning of industrial systems, the corrosion rate of steel increases rapidly. 

Waters of pH less than 6 may be expected to be corrosive, but, because any weak acids present in the solution may not be fully ionised, it does not follow that water of pH greater than 7 will not be corrosive. Mine waters are particularly corrosive to cast iron, often to such an extent as to preclude its use with them, because of their relatively high acid content, derived from the hydrolysis of ferric salts of the strong acids, mainly sulphate, and because the ferric ion can act as a powerful cathodic depolariser. 

When small amounts of impurities were added to concentrated phosphoric acid, the corrosion rate was not significantly affected. The addition of only 0-(X)7% ferric ion to concentrated hydrochloric acid caused only a slight increa.se in corrosion at 35°C, but at 1(X)°C the corrosion rate increased to 6-35 mm/y. 

High levels of chelant or oxygen affect the redox tendencies of iron-oxygen reactions and permit the liberation of Fe2+ ions (corrosion) from a metal surface and their subsequent chelation, thus preventing the formation or repair of blanketing ferric oxides, hydroxides, or a passivated magnetite film. 

C) for cast iron and up to 140 °F for marstenitic SS (60 °C). It is widely used where silicates are present with the iron oxides. Typically, 5 to 7.5% HC1 is employed. The ammonium bifluoride normally is present at 0.5%, but it may be increased to a maximum of 1.5% for a boiler that has not been cleaned for many years. The presence of hydrofluoric acid (HF), which is formed by the reaction of ammonium bifluoride with HC1 (see equation), tends to increase the rate of iron oxide dissolution and reduce the corrosion rate of exposed steel, when compared to using HC1 alone. This is due to the stability of the hexafluoroferric ion (FeFg3 ), which prevents the ferric ion from corroding exposed steel. 

NOTE Although the addition ofHF to HCl can be beneficial in helping to control the corrosion of steel (because fluoride ions form very stable self-limiting complexes with ferric ions), HF should not be used where significant hardness scales are present because calcium and magnesium fluorides (CaF2, MgF2) may be precipitated. 

Hydrochloric acid/stannous chloride. Modifications of the standard HC1 cleaning program to aid control corrosion of exposed steel include the addition of HF, as discussed earlier, but also stannous chloride. Where the cleaning program is likely to remove considerable volumes of rust and magnetite, even in the presence of a nitrogen- and sulfur-based proprietary corrosion inhibitor, rapid corrosion of exposed steel may develop. This is because the ferric ions (Fe3+) released from ferric oxide act to reduce the exposed steel to ferrous ions (Fe2+). 

One of the most ingenious ways in which corrosion is inhibited is to strap a power pack to each leg (just above the level of the sea) and apply a continuous reductive current. An electrode couple would form when a small portion of the iron oxidizes. The couple would itself set up a small voltage, itself promoting further dissolution. The reductive current coming from the power pack reduces any ferric ions back to iron metal, which significantly decreases the rate at which the rig leg corrodes. 

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. 

Flash rusting exhibited in neutral to alkaline water borne formulations appears to occur through a localised corrosion process probably Involving grit "activity" present from blasting, either directly or indirectly, in an electrochemical process. At such pH the rapid oxidation of ferrous to ferric ion produces... 

Ferrous Hydroxide A product of iron corrosion which is white in appearance, Fe(OH)2. When ferric ions enter, the product will appear black, brown, or green in color. 

In fact, when testing corrosion inhibitors, the major amount of corrosion may occur under the mounting washers as fresh inhibitor cannot reach such a confined space and, in addition, the low oxygen concentration leads to corrosion. As in the section above, ferric ions may seep from within the crevice and deposit around the fixture, thus giving rise to under-deposit corrosion. 

T. P. Hoar (38a) has suggested that the different behavior of ferric and ceric ions in silver dissolution may be considered from a purely electrochemical viewpoint. The anodic and cathodic potentials are very close in the ferric system their polarization curves meet at rather small values of the corrosion current, which does not require that all ferric ion near the metal surface be reduced to ferrous. On the other hand the potential of the ceric-cerous couple is about 0.8 V more noble, and complete reduction at the interface (or complete concentration polarization with respect to ceric ion) is necessary to lower this potential to the value of the Ag-Ag+ couple. 

Owing to the fact that free ferric ions are stable only in the acidic regime (Fig. 7.8), most slurries using ferric ions as an oxidizer are formulated at pH substantially below 4. At such a low pH, the copper surface oxidized via the reaction described in Equation 7.7 will not form any native oxide film. Without the protection from such an oxide, the copper surface is prone to corrode, which results in high static etch rate and practically no planarization efficiency. To provide a balance, therefore, the presence of a corrosion inhibitor is a must for copper CMP slurry. 

In the Purex process, plutonium and uranium are coextracted into an organic phase and partitioned by reducing plutonium(IV) to the aqueous-favoring plutonium(III). This has been achieved chemically by use of a suitable reductant such as ferrous sulfamate ( 1) or uranium(IV). (2, 3, 4, 5) The use of ferrous sulfamate results in accelerated corrosion of the stainless steel, due to the presence of ferric ions and sulfuric acid, and in an increase in the volume of wastes. The use of natural uranium(IV) can cause dilution of the 235U in slightly enriched uranium, thus lowering the value of the recovered uranium. 

Alloy B-3 can tolerate quite high concentrations of hydrochloric acid without severe corrosion, provided the solution is free of oxidizing ions such as Fe, Cu" ", Ni" " Mo" " , and Ti" ". In contrast, hydrochloric acid solutions containing these ions can be easily handled in vessels made of titanium, which would quickly corrode in pure hydrochloric acid. Even low concentrations (<30 ppm) of ferric ion in hydrochloric acid have been demonstrated to greatly reduce the corrosion rate of titanium. ... 

In the acidified anode channel containing HC1 and Fe3+, acid corrosion occurs with protons and ferric ions as oxidants ... 

If two or more electrochemical half-cell reactions can occur simultaneously at a metal surface, the metal acts as a mixed electrode and exhibits a potential relative to a reference electrode that is a function of the interaction of the several electrochemical reactions. If the metal can be considered inert, the interaction will be between species in the solution that can be oxidized by other species, which, in turn, will be reduced. For example, ferrous ions can be oxidized to ferric ions by dissolved oxygen and the oxygen reduced to water, the two processes occurring at different positions on the inert metal surface with electron transfer through the metal. If the metal is reactive, oxidation (corrosion) to convert metal to ions or reduction of ions in solution to the neutral metal introduces additional electrochemical reactions that contribute to the mixed electrode. 

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