Copper & copper alloys have two main corrosion failure modes – general corrosion & localised corrosion. The alloys suffer from other failure modes such as dezincification (selective de-alloying) & stress corrosion cracking. The basic principles of corrosion are dealt with in the Amtec Corrosion Guide & will not be covered here.
General Corrosion
The general corrosion rates for copper & copper alloys are generally low in unpolluted atmospheres. Indoors they only suffer from a light tarnish film that is associated with absolutely minimal thickness loss. However in more polluted atmospheres ( especially those containing ammonia compounds) corrosion rates can be much higher as the copper ion is complexed and can be washed away as a soluble species. Many copper alloys have good resistance to sea water & can be used successfully in marine exposure conditions.
Pitting Corrosion
Like most engineering metals both copper & its alloys can suffer from pitting corrosion in adverse circumstances. Copper piping used in central heating systems can pit very rapidly in certain waters unless steps are taken to prevent the early pit initiation stages. Pitting corrosion of all copper alloys has a pit initiation period during the initial service life. If the environment is stagnant or only flowing very slowly then the susceptibility to pit initiation will be much higher. Conditioning the surface in the initial stages is a good strategy for all copper alloys.
Stress Corrosion Cracking
Some copper alloys are susceptible to stress corrosion cracking, This type of breakdown occurs when components have either residual stress from the forming operation or are exposed under conditions where they are under a stress much less than would normally be required for the metal to deform or yield. Stress corrosion cracking only occurs in certain environments over particular temperature ranges. Exposure in environments that contain ions that can complex copper should be avoided unless a stress corrosion cracking resistant alloy has been selected.
Galvanic Corrosion
Galvanic corrosion occurs when two metals are connected together in an ionically conducting environment. One of the two metals becomes the anode (and dissolves) the other becomes the cathode and allows oxygen to be reduced (and become more alkaline) on its surface. Copper & copper alloys will corrode or dissolve much more rapidly if they are connected to passive stainless steel such as types 316 or 304 or Titanium or its alloys. Copper & copper alloys will cause galvanic corrosion of less noble metals such as low alloy steels stainless such as 410, cast iron, mild steel, aluminium, zinc & magnesium.
With all galvanic couples there is an area effect which is very important in determining the rate of corrosion – a large copper cathode will drive very rapid corrosion in a small Iron anode.
Deposition Corrosion
Deposition corrosion is a sub-set of galvanic corrosion where the cathode is caused by copper ions from solution depositing out on a metal lower down the electrochemical series such as steel or Aluminium. When this happens copper plates out of solution and forms a good local cathode.
De-alloying (Dezincification)
Copper alloys (especially some brasses) are prone to a type of de-alloying known as dezincification. This results in the surface of the alloy loosing zinc by elective dissolution. A gold coloured brass can turn a rich red colour when this happens due to the enrichment of copper in the surface layers. When the process takes place in severe environments the structure can be weakened to the point of decomposition into a spongy mass. There are two main types of brasses – alpha brasses (which contain less than 30% zinc) and alpha-beta brasses (which generally contain more than 30% zinc). The higher zinc content brasses, such as Muntz metal (60/40 brass), are much more susceptable to de-alloying, or dezincification, than the alpa brasses such as 70/0 brass. Dezincification has caused the fracture shown at the top of the page.
Cavitation Attack
Cavitation attack occurs most commonly on ship’s propellers & the impellers of high speed centrifugal pumps. It is not true corrosion but happens as a result of small vacuum filled holes or cavities imploding back on the metal surface & mechanically disrupting the normally protective oxide films on the surface. Specially developed cavitation resistant alloys are used in applications where cavitation attack can occur. Typical alloys used in circumstances where cavitation can be a problem are manganese bronze (otherwise known as high tensile brass), manganese aluminium bronze & nickel aluminium bronze.
Microbially-Induced Corrosion (MIC)
All copper alloys are highly resistant to Microbial corrosion as copper ions are natural biocides. If microbial corrosion is occurring elsewhere in the environment an producing local acidity from sulphurous acids then there may be some secondary effects.
Selective Weld Corrosion
Weld corrosion or weld grooving on copper alloy piping is another form of localised corrosion that can occur (especiallly in marine exposure conditions). It is a sub division of pitting corrosion where the anode has become fixed at the weld or the heat effected zone because of local changes in microstructure, local changes in composition or because of residual stress from the welding process. Copper nickels, & Aluminium Bronzes can suffer from this form of attack.
Stray Current Attack
Stray current corrosion can occur on copper pipes internally or externally when they are either buried or immersed close to a source of stray current strong enough to induce eddy currents in the pipe. These in turn under go a process of faradic rectification which results in a direct current polarisation of parts of the surface causing then to become localised anodic sites. Malfunctioning ICCP (Impressed Current Cathodic Protection) systems can have a similar effect.
Cathodic Protection
Both general corrosion & localised corrosion of copper & copper alloys can be controlled by using cathodic protection (see elsewhere on this site). This strategy is particularly successful in the case of adverse galvanic couples. Both ICCP & sacrificial anodes can be used. Sacrificial Anodes can be of Iron (or Steel), zinc or Aluminium. Magnesium anode can be very rapidly consumed when coupled to copper alloys due to the high difference in potential.
Numbering Systems
Several numbering systems are currently in use for copper & copper alloys. There are American, European & International numbering systems. Commonly the traditional names such as Naval Brass & Aluminium Bronze are still employed.