Iron oxides, corrosion

Internal treatment-related problems may take the form of organic material present in deposits of iron oxide corrosion debris and salt scales. The material typically is present as carbonized organic components and may originate from water treatment chemicals such as quebracho, wattle, pymgallol, or other tannin derivatives. Also, acrylates, starches, sulfonated lignins, and other sludge dispersants may be present. 

Notwithstanding the seriousness of these pre-boiler problems, however, it is material (especially iron oxide corrosion debris) originating in the condensate system, then transported back to the boiler itself, which carries the greatest risks of long-term operational problems. 

They also provide useful corrosion inhibition by the adsorption of calcium phosphonate onto iron oxide corrosion products, thus reducing the ferrous metal corrosion rate. Phosphonates can be described as cathanodic corrosion inhibitors. 

Oxygen corrosion in condensate pipelines is recognizable as large pits and are a typical result. Where the condensate pH level is low (say, due to the presence of carbonic acid), the pits may be particularly large, but as the pH level rises (say, due to the use of neutralizing amines), the layer of iron oxide corrosion product becomes more protective and the resulting pits tend to be smaller. 

Very visible form of corrosion in which voluminous layers form of brittle, iron oxide corrosion debris, usually covering a pit or deep crevice. 

We next consider metallic iron whose exergy reference species are oxygen molecules in the atmospheric air and solid iron oxide Fe203, which is the most stable existence of iron in the top layer of the lithosphere. In the atmospheric air metallic iron reacts with oxygen gas to form iron oxide (corrosion of metallic iron). The reaction at the standard state (unit activity, standard pressure 101.3 kJ, and standard temperature 298 K) is expressed in Eq. 10.30 ... 

Asami, K., Hashimoto, K. Shimodaira, S. (1976). X-ray photoclectron spectrum of Fe state in iron oxides. Corrosion Science, 16, 35-45. 

They can produce deposits which, when viewed under a microscope, are characterized by masses with moving microorganisms in the shape of spheres, rods, chains and vibrios, etc. They may produce slime, deposits of iron oxides, corrosion and acidification (H2SO4, HNO3, etc.). 

Corrosion is the gnawing away of materials due to exposure to different environments. Basically, a material is trying to return to its natural state, e.g., metallic iron oxidizes to fonn tire ore from whence it came. 

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. 

Internal surfaces exhibited many rounded, mutually intersecting pits partially buried beneath silt, iron oxide, and sand deposits. Orange and brown corrosion products and deposits overlaid all. Sulfides were present in the deposits and corrosion products. The material was easily removed when acid was applied (Figs. 4.21 and 4.22). 

Corrosion products contained sulfur up to 5% by weight. Aggressive sulfur- and chlorine-containing species concentrated beneath iron oxide, silt, and sand deposits. Localized areas of attack resulted. 

Internal surfaces of all tubes were severely attacked (Fig. 4.29). A brown deposit layer consisting of magnetite, iron oxide hydroxide, and silica covered all surfaces. Deposition was thicker and more tenacious along the bottom of tubes. These deposits had a distinct greenish-blue cast caused by copper corrosion products beneath the deposit. Underlying corrosion products were ruby-red cuprous oxide crystals (Fig. 4.29). Areas not covered with deposits suffered only superficial attack, but below deposits wastage was severe. 

Close visual examination of internal surfaces using a low-power stereomicroscope revealed a coating of reddish iron oxides on the internal surface. A population of small, knoblike mounds of corrosion product resembling tubercles was present on the surface (Fig. 5.16). 

Sulfides are intermixed with iron oxides and hydroxides on carbon steels and cast irons. The oxides are also produced in the corrosion process (Reaction 6.6). Although theoretical stoichiometry of 1 to 3 is often suggested between sulfide and ferrous hydroxide, empirically the ratio of iron sulfide to ferrous hydroxide is highly variable. Sulfide decomposes spontaneously upon exposure to moist air. Additionally, corrosion-product stratification is marked, with sulfide concentration being highest near metal surfaces. 

The weld was riddled with mildly undercut, gaping pits. Attack was confined to fused and heat-affected zones, with a pronounced lateral or circumferential propagation (as in Fig. 6.10). The resulting perforation at the external surface was quite small. Pits were filled with deposits, friable oxides, and other corrosion products. Black plugs embedded in material filling the gaping pit contained high concentrations of iron sulfide. Bulk deposits contained about 90% iron oxide. Carbonaceous material was not detected. 

Chemical analysis showed that each organism contained up to 50% silica by weight. Each was coated with iron oxides, silt, and other deposits and corrosion products. In places, large deposit accumulations were clearly correlated with large numbers of organisms. 

Normal mill coolant pH was near 5. The upset caused large amounts of iron corrosion products to be swept into the coolant. Settling of iron oxides and hydroxides fouled many mill components. 

Recent failures of the type illustrated in Fig. 12.21 affected a total of eight tubes in this condenser. Metal loss occurred exclusively on the top and bottom internal surfaces. Affected areas have a rough, jagged contour of deep, overlapping pits that were essentially free of corrosion products. Unaffected areas of the internal surface are smooth and are covered with a layer of black iron oxide. 

Graphitically corroded cast irons may induce galvanic corrosion of metals to which they are coupled due to the nobility of the iron oxide and graphite surface. For example, cast iron or cast steel replacement pump impellers may corrode rapidly due to the galvanic couple established with the graphitically corroded cast iron pump casing. In this or similar situations, the entire affected component should be replaced. If just one part is replaced, it should be with a material that will resist galvanic corrosion, such as austenitic cast iron. 

Internal surfaces were coated with a thin layer of reddish iron oxides. Significant corrosion was not observed on this surface. 

Figure 17.8 The black outer covering is corrosion product the reddish-hrown surface is coated with air-formed iron oxide.

Most of the surface is covered with a black corrosion product that is thicker in relatively low-flow areas near the hub. This layer of soft corrosion product can be shaved from corroded surfaces. Microstructural examinations revealed flakes of graphite embedded in iron oxide near the surfaces. 

Figure 17.10 shows metal loss on the throat of the pump housing. External pump housing surfaces were also affected (Fig. 17.11). Note the large tubercles. (Tubercles are knoblike mounds of corrosion products. They typically have a hard, black outer shell enclosing porous reddish-brown or black iron oxides) (see Chap. 3, Tuberculation ). The metal surface beneath these tubercles had sustained graphitic corrosion, in some cases to a depth of Vi in. (0.6 cm) (Fig. 17.12).

The lifetime of a conventional exhaust system on an average family car is only 2 years or so. This is hardly surprising - mild steel is the usual material and, as we have shown, it is not noted for its corrosion resistance. The interior of the system is not painted and begins to corrode immediately in the damp exhaust gases from the engine. The single coat of cheap cosmetic paint soon falls off the outside and rusting starts there, too, aided by the chloride ions from road salt, which help break down the iron oxide film. 

A frequently cited example of protection from atmospheric corrosion is the Eiffel Tower. The narrow and, for that age, thin sections required a good priming of red lead for protection against corrosion. The top coat was linseed oil with white lead, and later coatings of ochre, iron oxide, and micaceous iron oxide were added. Since its constmction the coating has been renewed several times [29]. Modern atmospheric corrosion protection uses quick-drying nitrocellulose, synthetic resins, and reaction resins (two-component mixes). The chemist Leo Baekeland discovered the synthetic material named after him, Bakelite, in 1907. Three years later the first synthetic resin (phenol formaldehyde) proved itself in a protective paint. A new materials era had dawned. 

In addition, with high solid content of the cooling water and at high flow velocities, severe corrosive conditions exist which continuously destroy surface films. Cathodic protection alone is not sufficient. Additional measures must be undertaken to promote the formation of a surface film. This is possible with iron anodes because the anodically produced hydrated iron oxide promotes surface film formation on copper. 

Rust—a corrosion product consisting mainly of hydrated iron oxide the term is used to describe the corrosion products of iron and ferrous ions. 

The carbon dioxide produced can contribute to the corrosion of metal. The deposits of ferric hydroxide that precipitate on the metal surface may produce oxygen concentration cells, causing corrosion under the deposits. Gallionalla and Crenothrix are two examples of iron-oxidizing bacteria. 

The presence of shell fouling affects the corrosion of steel structures in the intertidal zone where it has been found that the rust formed consists of irregular layers or iron oxides and lime, the latter accounting for up to 15% by weight of the corrosion product". The corrosion rate of mild steel in UK waters for the full immersion and intertidal zone is typically 0.08 mm/y compared with 0.1 to 0.25 mm/y in the splash zone according to the strength of wave action. Above the splash zone corrosion diminishes rapidly to 0.05-0.1 mm/y". 

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