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I'm in the process of looking for mufflers for my 2003 C4 Cab Tip. Some mufflers have Gillet stamped on them and others do not. I've been told that the Gillet mufflers are stainless steel. Are they indeed? If so why do they usually have their bottom seams corrode out after 80000-100000 km?

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Salt water causes stainless to "corrode" lose it's passive layer. 

 

http://www.bssa.org.uk/faq.php?id=9

 

What forms of corrosion can occur in stainless steels?

The most common forms of corrosion in stainless steel are:

  1. Pitting corrosion - The passive layer on stainless steel can be attacked by certain chemical species. The chloride ion Cl- is the most common of these and is found in everyday materials such as salt and bleach. Pitting corrosion is avoided by making sure that stainless steel does not come into prolonged contact with harmful chemicals or by choosing a grade of steel which is more resistant to attack. The pitting corrosion resistance can be assessed using the Pitting Resistance Equivalent Number calculated from the alloy content.
  2. Crevice corrosion - Stainless steel requires a supply of oxygen to make sure that the passive layer can form on the surface. In very tight crevices, it is not always possible for the oxygen to gain access to the stainless steel surface thereby causing it to be vulnerable to attack. Crevice corrosion is avoided by sealing crevices with a flexible sealant or by using a more corrosion resistant grade.
  3. General corrosion - Normally, stainless steel does not corrode uniformly as do ordinary carbon and alloy steels. However, with some chemicals, notably acids, the passive layer may be attacked uniformly depending on concentration and temperature and the metal loss is distributed over the entire surface of the steel. Hydrochloric acid and sulphuric acid at some concentrations are particular aggressive towards stainless steel.
  4. Stress corrosion cracking (SCC) - This is a relatively rare form of corrosion which requires a very specific combination of tensile stress, temperature and corrosive species, often the chloride ion, for it to occur. Typical applications where SCC can occur are hot water tanks and swimming pools. Another form known as sulphide stress corrosion cracking (SSCC) is associated with hydrogen sulphide in oil and gas exploration and production.
  5. Intergranular corrosion - This is now quite a rare form of corrosion. If the Carbon level in the steel is too high, Chromium can combine with Carbon to form Chromium Carbide. This occurs at temperatures between about 450-850 deg C. This process is also called sensitisation and typically occurs during welding. The Chromium available to form the passive layer is effectively reduced and corrosion can occur. It is avoided by choosing a low carbon grade the so-called 'L' grades or by using a steel with Titanium or Niobium which preferentially combines with Carbon.
  6. Galvanic corrosion - If two dissimilar metals are in contact with each other and with an electrolyte e.g. water or other solution, it is possible for a galvanic cell to be set up. This is rather like a battery and can accelerate corrosion of the less 'noble' metal. It can avoided by separating the metals with a non-metallic insulator such as rubber.
Edited by fpb111

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Thanks for the information. I take it that the Gillet mufflers are stainless steel and under the circumstances that you have outlined, they can pit and corrode. The seam that appears most vulnerable is the bottom one. Probably due to start up moisture being dependent and its mixture with contaminants from within. I guess external salt and water in winter conditions would also contribute.

Just for the record, if you wish to buy used mufflers, look at the bottom seams VERY carefully. Also even if they appear intact, you still do not know what's happening within. Southern states and low mileage may be of equal importance..

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