Cast iron oxygen corrosion

Tubercles are mounds of corrosion product and deposit that cap localized regions of metal loss. Tubercles can choke pipes, leading to diminished flow and increased pumping costs (Fig. 3.1). Tubercles form on steel and cast iron when surfaces are exposed to oxygenated waters. Soft waters with high bicarbonate alkalinity stimulate tubercle formation, as do high concentrations of sulfate, chloride, and other aggressive anions. 

In oxygenated water of near neutral pH and at or slightly above room temperature, hydrous ferric oxide [FelOHla] forms on steel and cast irons. Corrosion products are orange, red, or brown and are the major constituent of rust. This layer shields the underl3dng metal surface from oxygenated water, so oxygen concentration decreases beneath the rust layer. 

Oxygen corrosion only occurs on metal surfaces exposed to oxygenated waters. Many commonly used industrial alloys react with dissolved oxygen in water, forming a variety of oxides and hydroxides. However, alloys most seriously affected are cast irons, galvanized steel, and non-stainless steels. Attack occurs in locations where tuberculation also occurs (see Chap. 3). Often, oxygen corrosion is a precursor to tubercle development. 

Carbon steel heat exchangers, cast iron water boxes, screens, pump components, service water system piping, standpipes, fire protection systems, galvanized steel, engine components, and virtually all non-stainless ferrous components are subject to significant corrosion in oxygenated water. 

The corrosivity of a natural water depends on the concentration and type of impurity dissolved in it and especially on its oxygen content. Waters of similar oxygen content have generally similar corrosivities, e.g. well-aerated quiescent sea-water corrodes cast iron at ratesof 0 05-0-1 mm/y while most well-aerated quiescent fresh waters corrode iron at O Ol-O-1 mm/y. 

While well-aerated near-neutral waters are normally much more corrosive than poorly-aerated waters, waters with near zero oxygen contents may cause high rates of corrosion if active sulphate-reducing bacteria, which can act as very efficient depolarising agents, are present. A corrosion rate of 1 5 mm/y has been observed on cast iron exposed to such a water. 

Information about the corrosion of boiler piles comes from analysis of scale samples and also from laboratory experiments. Smith and McEaney (1979) used XRD and SEM to follow the initial stages in the development of scale on gray, cast iron in water at 50 °C. At first, the corrosion product was a mixture of magnetite and green rust. Whether lepidocrocite formed depended on the level of oxygen in the system. 

This is yet a further type of concentration cell corrosion and is often found in heat-exchanger box ends made of cast iron or mild steel end-covers. The pattern of events is, as before, with some initial general oxygen corrosion,... 

As mentioned previously, oxygen controls the corrosion rate in the neutral pH range, if the oxygen is less than 1 ppm, the penetration rate in carbon steel or cast iron will be less than 1 mpy (0.025 mm/y) at room temperature, provided no corrosive bacteria are present. If corrosion-inducing bacteria are present, treatment with a biocide such as chlorine is imperative. In theory, because freshwater can be treated, carbon steel exchanger tubes can be used. However, control of water treating equipment is sufficiently difficult and... 

A metal pipeline was unlikely to be satisfactory in this application. Cast iron would be expected to last perhaps 20-30 years, whereas the design life was to be 50 years. It would be subject to internal attack from the effluent and external attack from the sea water. Protection against this type of corrosion would be difficult. Similar objections apply to use of mild steel, where again predictable protection would be difficult to achieve mild steel would also suffer rapid internal corrosion. A suitable grade of stainless steel could probably be selected to resist internal attack by the effluent but the problem of external corrosion would remain. Stainless steel is subject to corrosion pitting in sea water, especially when the oxygen content is low. In quiet water the rate of pitting can be 6.9 mm per year. 

The corrosion rates of these materials in almost neutral waste waters are chiefly determined by the concentration of oxygen and by its transport to the surface of the material. However, in addition to uniform surface attack, which must be taken into account by increasing the wall thickness, there may be increased local attack in the form of shallow pit corrosion and pitting corrosion due to the formation of aeration cells. The use of unalloyed and low-alloy steels as well as cast iron and cast steel is generally not recommend if there are no additional corrosion protection measures, e.g. with coatings, linings, or cathodic protection. 

The rate of corrosion of cast iron is directly linked to oxygen content. Carbon dioxide accelerates the formation of scales in fresh water. Cast iron and carbon steels are sensitive to attack by hydrogen sulfide even in the absence of oxygen. Corrosion rates of low carbon steels and cast iron accelerate with velocity, if the water is not treated with inhibitors. 

The type of anode material has an important effect on the reactions encountered on the anode surface. For consumable metals and alloys such as scrap steel or cast iron, the primary anodic reaction is the anodic metal dissolution reaction. On completely passive anode surfaces, metal dissolution is negligible, and the main reactions are the evolution of gases. Oxygen can be evolved in the presence of water, whereas chlorine gas can be formed if chloride ions are dissolved in the electrolyte. The reactions have already been listed in the theory section of this chapter. The above gas evolution reactions also apply to nonmetallic conducting anodes such as carbon. Carbon dioxide evolution is a fiirther possibility for this material. On partially passive surfaces, both the metal dissolution and gas evolution reactions are important. Corrosion product buildup is obviously associated with the former reaction. 

---