Corroded bolts

31st July 2009

Corroded bolts

Initial Report

CHIRP Narrative: In Maritime FEEDBACK No.22, we published a report from a ship manager in which the stainless steel bolts securing a lifting lug on a rescue boat had failed.  This occurred when the boat was being swung out.  Two of the crew fell 12 metres into the sea, fortunately without major injury.  We have received letters suggesting possible causes of the corrosion. Here are a selection:

Corroded Bolts (1)

Letter Text: The failure of the stainless steel bolts in the rescue boat was almost certainly due to anaerobic corrosion.  I have had previous experience of this type of failure and it is probably more common than would be supposed.  One case was of the failure of 12mm bolts holding down a radar support tripod.  This tripod became rather loose and when I removed the head lining inside the boat two of the nuts with part of the bolt shanks still in place fell on the deck.  The third one was still sound.

Anaerobic corrosion occurs when stainless steel is subject to sea water seepage but is deprived of a supply of fresh air.¬† This can be the situation with through bolts, particularly in GRP, if the bolts fit tightly but do not entirely exclude sea water.¬† The bolts that didn’t fail were probably well sealed against the ingress of water.

The condition can occur in stainless wire rigging where the outer strands appear to be quite sound but the inner strands fail due to this type of corrosion.

The real problem is that there are no outward signs and most installations are never affected but in some cases, such as the one you report, such failure could be disastrous.

Corroded Bolts (2)

Letter Text: I recall that many years ago we had some incidents of corrosion of stainless steel, including corrosion of a propeller shaft under a wrapping.  It was concluded that the cause of this corrosion was anaerobic corrosion of stainless steel, which occurs where air is excluded from the surface of the stainless steel.  It seems possible to me that this was a factor in this case, although I note that the problem did not seem to be present on the other boats examined.

Corroded Bolts (3)

Letter Text: I have been reading your CHIRP Maritime Feedback issue no. 22 and my attention was caught by the report on the Rescue Boat Accident.  The photograph of the fractured bolts (photo 3) reminded me of three failures which came to my attention when I was a Lecturer at a University.

The failures all related to high tensile stainless steel (HTSS) either in the form of chain or nuts, bolts and washers.  In each case the HTSS had been used in conjunction with galvanized components and exposed to sea water or at least a marine environment.  The chain had been used with ordinary galvanized shackles, the bolts to secure a galvanized backing plate and in one case the bolts themselves had been galvanized. All failures occurred suddenly, without warning, with fractures which showed no accompanying stretching or deformation.  I was able to reproduce these failures in the laboratory by loading a tensile test piece overnight.  The test piece was in contact with a scrap of zinc held in place by a gauze bandage soaked in sea water.  When the test piece was subsequently stretched a network of cracks was revealed and the ultimate fracture resembled those of the failed components.  If I omitted either the zinc or the sea water bandage, the material behaved normally stretching and necking down to a normal fracture.

To return to the failures you illustrate you do not give the material of the eye-plate but from the position of the fractures the problem would appear to lie under the decking of the rescue boat and one wonders whether the bolts had been married with either galvanized washers or nuts or perhaps a galvanized backing plate.  You report that other bolts and eye-plates have been inspected and found to be satisfactory.  I would warn that the type of cracking which I observed is difficult to detect non-destructively.  I was able to observe it by metallographic examination but the cracks are very fine and could easily escape detection.

From time to time I hear of similar failures and it is clear that the dangers of marrying HTSS with galvanized components in a marine environment is not fully appreciated and requires greater publicity.¬† I suspect that designers who appreciate the benefits of ordinary 18/8 stainless steel are mesmerized by the words “high tensile” and do not appreciate that the two materials differ significantly.

CHIRP Comment:

Although we have not been informed of the actual failure mode of the bolts in the rescue boat reported in Maritime FEEDBACK No.22, it is very credible that this was due to crevice corrosion caused by exposing the bolt to anaerobic conditions.  Some grades of stainless steel are more prone to this type of corrosion and potential subsequent failure.  Operators should always ensure that when selecting materials they are suitable for the conditions to which they are exposed.

It is also credible that, if a backing plate of a different material had been used, galvanic conditions could have been generated resulting in corrosion and subsequent failure of the assembly.  The use of different metallic materials should be avoided.

We have also received a report from another ship manager of an incident in which the lifting eye of a glass reinforced plastic rescue boat pulled out, not through failure of stainless steel but because the surrounding GRP material failed. We will publish the full report in the next issue.

 

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