Fasteners - hdgasa

Fasteners
Report on sample bolts from Medupi
On 20 September 2011 five sample
bolts and one short threaded rod was
received from Profab Engineering c/o
Medupi Power Station Lephalale. As
per your request our Association was
requested to review the quality of the
hot dip galvanizing. The samples
received were photographed as shown
in photo 1 (right).
After a general visual inspection the
steel from which the components were
manufactured indicate a reactive
material to molten zinc resulting in a
“rough” (photo 2) surface finish
producing a thick coating well in
excess of the SANS 121 (ISO
1461:2009) specification requirements
of a mean coating thickness 45µm.
Photo 1: Sample bolts and a short threaded rod received for a hot dip galvanizing quality review.
Actual measurements taken indicate
the following:
No. of readings taken 10
Maximum thickness 195µm
Mean coating thickness 123µm
Minimum thickness 49µm
The thicker than normal coating
thicknesses are due in part to the
rough surface finish. The extra zinc
coating thickness is an advantage
with regards to longer service life,
but could well introduce an assembly
problem during the application of the
nuts.
Two of the samples are not in
accordance with the requirements of
the specification as the threads were
found to be “clogged” with excess
zinc. These two components are
illustrated in photo 3 and are
unacceptable.
A previous photograph (photo 4) e-
mailed to us on 1 September
indicates corrosion products on the
bolt. We can speculate that the hot
dip galvanized thread may well have
been retapped in order to accept the
nut. Generally this is not accepted
practice as the quality of galvanized
threads should be able to accept
nuts without re-tapping.
Conclusion
In general, the quality of the hot dip
galvanized samples received; do not
represent normal standard quality.
Two reasons are suggested for this:
1. Steel chemistry and the reaction
with molten zinc.
2. Poor quality controls during the
hot dip galvanizing process.
Should these samples represent the
quality of the total consignment
there are grounds to reject and to
request remedial corrective action.
I trust that this report is a fair
representation of the samples
received. Should any additional
detail or explanation be required,
please refer to the undersigned.
R.E. Wilmot
Photo 2: Rough surface finish usually the result
of steel chemistry that increases the reactivity
when immersed into molten zinc.
Photo 3: Bolt threads not cleaned and as such
are not in conformance with the specification
Such components are rejectable and should be
returned to the responsible galvanizer.
Photo 4: Corrosion products evident on a bolt
thread after assembly. Causes could be retapping,
or poor quality galvanizing both of
which are unacceptable.
24 Hot Dip Galvanizing Today Volume 9 Issue 1 2012