Corrosion is often thought of as the oxidation of metals such as iron, but ceramics also corrode, or react with their environment. Concrete, for example, generally is very stable, but it contains calcium hydroxide and calcium aluminate, which are attacked by sulphates, such as calcium sulphate often present in ground water. Tungsten carbide, usually highly resistant to corrosion, is destroyed in less than a week of contact with sulphuric acid, H2SO4.
Concrete is used underground to protect steel smictures and pipes, as well as the reinforcing steel rods within concrete itself. When concrete sets, it generates alkalis that produce a ferric oxide film over the steel surface. The ferric oxide film protects the steel from further corrosion.
The structure of concrete is sometimes porous, allowing water to penetrate. Under certain conditions, the water may be acidic, attacking the steel rods. If calcium chloride is added to accelerate cement hardening, it may donate corrosive chloride ions. A dense, complete covering of concrete over the rods is usually sufficient to protect against corrosion.
Glass resists many chemicals, including detergents and acids. However, glass may crack when the temperature changes rapidly. It is also susceptible to fracture from impact. Glass liners can be broken even when not impacted directly.
Enamels are highly resistant to corrosion, protecting many steels and cast iron. Enamels are comprised of silicate and borosilicate glass, with the addition of fluxes to promote adhesion. For applications in contact with acids, a smaller amount of flux is used. Chemical equipment requires an enamel with a higher percentage of borosilicate glass.
The composition of the enamel should be adjusted so that its coefficient of thermal expansion closely matches that of the underlying metal. At least two coats of enamel are applied. The first coat, or ground coat, contains cobalt or nickel oxides (for coating steel) or lead oxide (for east iron) to aid adhesion.
Steel used in enamelware requires a low carbon content. The carbon may react with the molten oxides in the enamel to form a gas, which causes blistering of the enamel. Alloyed steels. may be used, such as those containing titanium. The titanium combines with all the carbon, nitrogen, and oxygen to produce inert compounds, preventing the formation of gases.
High temperature corrosion resistance is attained with silicide coatings. A thin layer of fused suicide is applied to metals such as a columbium alloy. However, the silicide coating is brittle and susceptible to chipping. Another high-temperature coating consists of a borosilicate glass matrix reinforced with iron and nickel powders. The iron and nickel impart toughness to the composite.
Chrome oxide is another corrosion-resistant ceramic. It has the ability to bond to glass, capable of repairing fractured glass linings of chemical tanks. It also resists erosion and abrasion. Other oxides include aluminum oxide and zirconium oxide. These oxides are resistant to a variety of chemicals, but are not recommended for long term exposure to strong acids or bases.
Tungsten carbide is widely used in wear applications. It resists strong bases, such as NaOH and KOH, even at higher temperatures. However, strong acids, such as HSO and HNO attack the cobalt and nickel binders in tungsten carbide, weakening its structure.
Silicone carbide resists both strong acids and strong bases. After 125 hours of submersion in HSO, it experienced a corrosive weight loss of only 0.21 x 103oz/in.yr (1.8 mg/cmyr) compared to 15 x 10 oz/in.2yr (65 mg/cmyr) for aluminum oxide, and more than 230 x 10oz/in.yr (1,000 mg/cmyr) for tungsten carbide. Silicon carbide is virtually unaffected by HF, HCL, and KOH, having a corrosive weight loss of less than 0.05 x 10oz,/in.2yr (0.2 mg/cmyr).
Testing and applications
Corrosion testing is performed by submersion of the material to be tested in a chemical bath containing the substance it is proposed to resist. The bath is stirred continuously for the duration of exposure, often 100 to 300 hours.
The weight before and after testing is compared to determine the weight loss. The surface is examined, and a comparison of the strength can be made, between parts that have been placed in the chemical bath and those that have not.
Ceramics are often applied to protect steel and east iron parts. Concrete guards steel structures and piping underground. Glass frequently lines valves, as well as water and chemical tanks. Enamel coatings are applied to steel and iron in chemical plants. Plumbing fixtures and appliances also are coated with enamel.
Silicide coatings are applied to rocket nozzles for high-temperature oxidation protection. Seals and bearings in contact with strong acids and bases are made of silicon carbide. Pump sleeves, shafts, beatings, and bushings are formed of silicon carbide to simultaneously protect against wear and corrosion.
Corrosion test results in liquids
Corrosive weight loss (mg/cm yr)**
carbide (6% Co)
(No free Si)
|10% HF plus||2577||>1000||>1000||16.0||<0.2|
*Test time: 125 to 300 hours of submersive testing, continuously stirred.
**Corrosion weight loss guide:
1000 mg/cm yr – Completely destroyed within days.
100 to 999 mg/cm2 yr – Not recommended for service greater than a month.
50 to 100 mg/cm2 yr – Not recommended for service greater than one year.
10 to 49 mg/cm2 yr – Caution recommended, based on the specific application. 0.3 to 9.9 mg/cm2 yr Recommended for long term service.
<.2 mg/cm2 yr – Recommended for long term service: no corrosion other than as a result of surface cleaning was evidenced. Courtesy, Carborundum