**8. Corrosion**

Corrosion in automotive fuels is normally caused by acids. An important distinction is that weak organic acids can be present in the crude oil itself and thus carried over to automotive fuels. However strong inorganic acids are not found in crude oils. These are used by various refinery processes and should be removed thereafter. Thus, detection of string acids is a clear sign of contamination while weak acids could originate from the crude oil.

Total acid number (TAN) by ASTM D974 is defined as the amount of milligrams potassium hydroxide (KOH) required to neutralize all acid compounds in 1 g of oil sample. This test determines the amount of soluble or nearly soluble acids in a sample dissolved in a toluene and 2-propanol mixture. The higher the TAN, the more corrosive the fuel possibly is. However, TAN does not distinguish between different acid strengths and further investigation related to the proportion of acids molecules which have dissociated into cations and anions could be required if a fuel is found to have high TAN.

Normally corrosiveness is both automotive fuels is measured by copper strip corrosion measured by ASTM D130. Corrosion can affect metallic components in vehicle fuel systems, dispenser pumps and fuel storage systems and is related to the fuels' acidity. The test procedure uses a strip of polished copper which is immersed in a sample of the fuel and heated to a stated temperature for a prescribed time. The degree of corrosion is measured by comparing the staining with a reference sample.
