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PLAIN BEARING FAILURE
6.0 Chemical effects
- Chemical effects Index
6.4 Tin Oxide Formation on White Metal
| Click on photograph to enlarge | |
 | Tin oxide deposit on trailing edge of thrust pad |
 | Tin oxide formation on thrust bearing, the deposit covering the complete surface of the pads. Bearing failure has caused melting of the underlying white metal causing cracking of the brittle tin oxide layer |
| Main Characteristics | Deposit was identified by x-ray diffraction as tin oxide. The deposit of tin oxide is black and brittle. This is shown by the patchy nature of the deposit in Photograph 'a' and the way the deposit has broken up when not supported by solid white metal, Photograph 'b'. (Rather similar to a thin layer of ice on mud, that fractures when stood on.} The particles of the deposit that have broken away in the case of the single pad (Photograph 'a') have scored the softer white metal in the direction of motion, though these scores do not extend to the trailing edge, probably because they have broken down to small particles of size below that of the oil film; this is suggested by the small radial 'ticks' at the ends of some of the score marks. |
| Cause | Tin oxide was a particular problem in marine steam turbines. Tin oxide forms by an electrochemical reaction of a tin-rich white metal and an electrolyte. This occurred in marine turbine sets as steps were taken to remove steam condensate from the oil. In the marine environment there was also some contamination by salt (sodium chloride) and as the oil was dried the concentration of the electrolyte increased until reaction occurred |
| Note | The problem has become much less common now that the condition of the oil in marine steam turbine lubrication systems has been improved, but tin oxide formation still occurs occasionally in systems with sea water cooling and machines operating in marine environments. |
| Possible Confusion with Other Types of Damage | Other deposits described in this Section tend to look superficially the same. The final diagnosis requires chemical analysis |
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