VISWA LAB TECHNICAL UPDATE:
Presence of Lead in Stern Tube Lubrication Oils
We have received two cases of elevated and increasing lead levels in stern tube lubricating oils. The concern is, if the stern tube bearings (which may contain lead alloys) are wearing out. If such is the case, the matter is of deep concern.
There are quite a few suppliers of EAL (Environmentally Acceptable Lubricants) products in the market. Some of these products are ester based, which has caused the concern that the ester may be breaking up into acids and alcohols. The acids elevate the acid content in the lube oils resulting in acidic corrosion of the stern tube bearing material. At‐least one manufacturer of marine lubricating oils has reported carrying out more detailed investigation into this problem. His conclusion from the investigation are
- The oil supplied by him doesn’t contain any Lead.
- If the Lead noticed in the lubricating oil is a wear metal, then the analysis should also show wearing of other bearing metals forming part of the alloy such as Tin or Antimony and this has not been the case.
- Even Tin based alloy bearings with near‐zero Lead are showing a spike in Lead content in the oil.
The conclusion drawn by this supplier is that, the increase in Lead levels have nothing to do with lubricant condition or the acid content of the used lubricant.
This supplier has quoted the findings of an investigation carried out by Lloyd’s register marine technical dept. The conclusion is that there is no selective corrosion or leaching of the white metal bearing material in the stern tube.
So where is lead coming from?
The structural parts of the tanks which are in contact with stern tube system may have been coated with red‐lead anti corrosive primer paint during fabrication. The lubricant coming in contact with such paint may be receiving the leached lead from the paint.
How to check if this hypothesis is correct?
We suggest you test the stern tube lubricating oil every month and carry out a trend analysis study. If no other element is leached except Lead, perhaps it doesn’t represent a bearing wear.
If it is the leached lead from the paint, the spike in the lead values will increase up to a stage after which the lead level will stabilize and not increase. This may indicate it is the lead from the paints that is coming into the lubricating oil.
Please do not hesitate to contact us with any questions.
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