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Process Chemicals Corrosion

Chlorine

The flow rate of liquid chlorine through carbon steel pipework is restricted to 2 m/s to avoid removing the ferric chloride coating on the pipe surface which protects against erosion / corrosion of the carbon steel. Wet chlorine gas corrodes mild steel. PVDF (preferably), ebonite, or rubber lined steel is used for this duty. Chlorine gas handled at temperatures in excess of 200C in carbon steel can result in chlorine / steel fires. Zinc can be used for this duty, but for low temperature (e.g. liquid) chlorine special steels are required to avoid embrittlement. Titanium is unsuitable for Chlorine duty and should be avoided.

Bromine

Susceptibility of materials of construction to attack by bromine is strongly dependent on the conditions of service including temperature, pressure and moisture content. Therefore, wherever possible materials selected for bromine duty should be tested under the actual conditions of use.

Storage vessels are commonly constructed of steel lined with lead, PVDF (and certain other fluoropolymers) or glass. If the bromine is 'dry' then Nickel or alloys such as Monel and Hastelloy can be used though all are susceptible to severe attack in the presence of wet bromine. Titanium is unsuitable for Bromine duty (wet or dry) and should be avoided.

Lead is used for lining steel storage vessels and less frequently for lining pipes but at high moisture contents and/or elevated temperatures the protective layer of lead bromide that forms on the surface of the metal is susceptible to degradation. Non-metallic linings including glass and certain fluorocarbon polymers, including PVDF and PTFE have replaced lead in most applications. Melt processed polymers such as PVDF, PFA and ETFE are preferred to PTFE due to its inherent porosity.

Few metals are suitable for use in contact with 'wet' bromine (moisture content in excess of 30mg/kg). Niobium, Tantalum and alloys of these two metals are suitable but high cost restricts their use (e.g. bursting discs and instrument components).

Sulfuric Acid

Corrosion protection of mild steel vessels occurs by the formation of an iron sulfate coating. Any condition leading to excessive turbulence can result in the removal of the coating and corrosion. Accelerated corrosion can also occur at air/acid interfaces due to interfacial dilution. Additionally the temperature influence on corrosion rate varies with different strengths of acid and consequently it is necessary to define maximum operating temperatures. Chemical lead is sometimes used where steel is unsuitable and PVC or fluorocarbon plastics can be used in certain applications. Specially developed stainless steels have replaced traditional cast iron applications for high temperature duties.

Hydrochloric Acid

This acid is very corrosive towards most of the common metals and alloys. This is exacerbated where aeration or contamination by oxidising agents is present. Copper is particularly prone to this problem. Also many failures occur due to the presence of minor impurities such as ferric chloride. Plastics and rubber-lined steel are widely used for pipework and small vessels.

Ammonia

Materials of construction for ammonia are dependent on the operating temperature. Whilst mild steel may be used at ambient temperature special steels are required at low temperatures to avoid embrittlement. Impurities in liquid ammonia such as air or carbon dioxide can cause stress corrosion cracking of mild steel. Ammonia is highly corrosive towards copper and zinc.

Hydrofluoric Acid

Bulk storage of 70% acid or greater may be in mild steel or PVDF tanks. Polyethylene, polypropylene, and PVDF are commonly used for construction of major components. PTFE is often used for smaller components such as gaskets. Glass or GRP should never be used.

Oxygen

Materials suitable for liquid oxygen service are nickel steel, austenitic stainless steels, and copper or aluminium alloys. Carbon steels and plastics are brittle at low temperatures and should not be used on liquid oxygen duty. PTFE is the most widely used sealant.

Hydrogen

At temperatures below 120C carbon steel can be used up to high pressures. At elevated temperatures and significant pressures hydrogen will penetrate carbon steel and react with the carbon to form methane. This results in a loss of ductility and cracking or blistering of the steel. For high temperature applications steel alloys containing molybdenum and steel are satisfactory.