Symptoms

The main symptom of the “Corrosive sulphur from sulphur combustion by-products (C3)” criticality is related to the presence of corrosive sulfur compounds as by-products of the combustion of fuller’s earth (see causes).

The main diagnostic indicator for this criticality is
 TCS – Total Corrosive Sulfur (IEC 62697-2)
Total corrosive sulfur can be expressed as sum of all corrosive sulfur compounds or as a concentration of DBDS equivalent. If the TCS concentration, expressed as DBDS equivalent, exceeds the recommended values (see table below in the section on diagnosis), the required treatments must be performed.

There are other co-factors useful for completing the diagnostic picture:

Potentially corrosive sulfur – CCD Test (IEC 62535)
Corrosive sulfur (IEC 62535, ASTM D1275 Method B, DIN 51353)
Additives:Passivators (BTA, Irgamet 39, Irgament 30); oxidation inhibitors (DBPC, DBP)
Elemental sulfur

DBDS analysis methods are unable to determine the corrosiveness of sulfur compounds responsible for the “Corrosive sulphur from sulphur combustion by-products (C3)” criticality.
To determine total corrosive sulfur, in particular that not due to DBDS, Sea Marconi has invented, developed, industrialised (and patented No. 0001394617 of 2008) the method called TCS – Total Corrosive Sulphur. This analytical technique is independent of individual corrosive compounds, but evaluates the effects equivalent to DBDS in terms of the amount of copper sulfide produced (under the same test conditions).
This method will be included in the IEC 62697 standard, Part 2 “Test Methods for Quantitative Determination of Total Corrosive Sulphur (TCS)” currently in the Committee Draft for Voting (CDV) phase. The round robin tests performed were excellent and formed the basis for the IEC working group.

Development of this method has experimentally shown that the conversion of the different sulfur compounds into total corrosive sulfur (TCS) occurs differently depending on the temperature and the molecular characteristic of the compounds themselves.

Further support for understanding and diagnosing C3 criticality will be provided by the results of the working group on IEC 62697 “Part 3 – Test Methods for Quantitative Determination of Total Mercaptans and Disulfides (TMDs) and Other Targeted Corrosive Sulfur Species”, currently in the Draft Committee (DC) phase. For the time being, a round robin test has been completed on samples of regenerated oil, finding sulfur concentrations of over 100 mg/kg. The samples used are related to regenerated oils through reactivation of fuller’s earth and combustion.

M.C. Bruzzoniti, R.M. De Carlo, C. Sarzanini, R. Maina, V. Tumiatti, Stability and Reactivity of Sulfur Compounds against Copper in Insulating Mineral Oil: Definition of a Corrosiveness Ranking, Ind. Eng. Chem. Res., 2014, DOI: dx.doi.org/10.1021/ie4032814