The RFA group is well known for it’s expertise in electronic failures and environmental legislation but we also help customers from a range of industries who encounter corrosion in their products or processes. With the diverse expertise of our staff we are often able to identify the route causes of corrosion problems that may have been overlooked in initial investigations. A recent example highlighting this involved biological corrosion.

How biological corrosion arises

Biological corrosion occurs either directly or indirectly by the activity of living organisms on a metal. In the absence of oxygen, anaerobic bacteria in soil or water may reduce sulphates to sulphides, which accelerate corrosion and can be identified by the presence of metal sulphide deposits in the corrosion product. Alternatively, in aerobic conditions, when oxygen is present, bacteria may also oxidise elemental sulphur or sulphur bearing compounds to produce sulphuric acid at concentrations of up to 5wt%, thus creating extremely corrosive conditions. Other bacteria are capable of oxidising ammonia to cause an appreciable accumulation of nitric acid. In the right conditions biological corrosion can act at an alarming pace with corrosion rates up to 2mm per year!

Case study – rail system failure

RFA were asked to investigate why holes were formed in pressurised air coolers from rolling stock used in a demanding environment. On dismantling the coolers yellow deposits were evident on the surfaces of the aluminium vanes used to conduct air flow through the cooler. Beneath these deposits a series of small pits were observed, which in some locations had perforated the cooler wall, leading to a loss of pressure in the system.	Figure 1.Yellow deposits on aluminium cooling vanes
Figure 2. Pitting corrosion of the aluminium	Pitting corrosion will occur on aluminium when there is a breakdown in the stable oxide, allowing aggressive localised corrosion. Analysis of the yellow deposits within the cooler revealed that they contained a large amount of organic material with relatively high levels of sulphur (3-4 wt%).
Scanning electron microscopy (SEM) of the product deposited around the pits found forms that suggested bacteria may have been present. A combination of Ion chromatography and Fourier Transform Infra Red Spectroscopy (FTIR) of the corrosion products identified lubricant oil and high level of sulphate (substances known to stimulate bacterial growth). Further testing confirmed the presence of organo-sulphide in the yellow deposits.	Figure 3. Evidence of biological attack.

For confirmation a sample of the deposit was sent for biological culturing and micrococci bacteria were found to be present; these act aerobically to form sulphuric acid in environments containing sulphur. The oil used in these coolers was a natural mineral oil that would allow such bacteria to thrive. A switch to synthetic oil for the coolers was therefore recommended to alleviate the problem.

Further information
RFA is currently investigating further failures in copper heat exchanger pipes that also appear to have been influenced by the presence of bacteria. To find out more call us on +44 (0)1372 367444 or email.