Bolt coating.

In addition to Raghu's email, sorry that I forgot to mention that the
multilayer polymer coated bolts are being recommended for service
temperatures of less than 200 deg.c and for all material bolting
assemblies in highly corrosive atmospheric seashore site locations.

Just to pointout, additionally the zinc reacting with SS being
refered as the LME-Liquid Metal Embrittlement effect. Which is true
for metal coated SS bolting and painting material having zinc as an
ingredient which can possible spill over during site painting
acitivity and during fabrication or erection welds crossing the
melting temp. of zinc which will lead to LME. Accidents of failure of
massive pressure equipments have occured due to this minor mistake of
paint spray on SS vessels before going for heat treatment in shops.
Moreover this embrittlement moves across the thickness.

With regards,
Kannan.

Bathula Raghuram wrote

Zinc- or cadmium-coated steel fasteners shall not be used for
applications above 400°F: these coatings may cause hydrogen embrittlement.
Especially avoid mixing zinc- and cadmium-coated nuts, bolts, or washers at
temperatures above 300°F: the zinc and cadmium will melt and mix. The
resulting mixture is known to cause intergranular cracking, with subsequent failure of the fasteners in a short time. Failure of fasteners as described above can result in serious injury to personnel and damage to equipment. Cadmium or zinc is not permitted where it would be in
contact with fuel oil, lubricating oil, grease, or petroleum-based hydraulic fluid. This restriction does not prohibit the use of cadmium or zinc plated fasteners in locations that are external to these systems if there is no danger of contaminating the working fluid. For example, cadmium
or zinc plated fasteners could be safely used as hold down or mounting bolts for a hydraulic control valve since there is no danger of contact between the external fasteners and the fluid inside the valve. Personnel should wash their hands after handling cadmium plated fasteners to avoid ingesting cadmium.

Kannan Sundaram wrote:

Felt to caution people who prescribe the bolt coating.

In an inhouse test of Polymer coated bolt of of a reputed Japanese vendor, which was to perform 4000 hours of salt spray test as per B 117, did not meet the requirement though the vendor's test certificates prooved so. The inhouse test was performed as in one of the plant it was used and has peeled off while normal wrenching and never stood even 1/4 of the life period gauranteed by the supplier. Literally all bolting of the plant had to be replaced by the client.

The final recommendation is that it is ideal to have a phosphating coat and a galvanizing or zinc coat and the final coat to be fluorocarbon polymer coat which really stood well in acc. with B117 in the inhouse test and has been proven in few operational plants.

So as a caution note, it would be a good practice to go for multiple coat rather than single coat for the general call for such coating requirements.

I do not want to name the companies explicity in this list for obvious reason.

Readers' experienced views are welcome.

With regards,
Kannan.