8
B ULK D ISTRIBUTOR
Cleaning & Repair
November/December 2019
The pitfalls in pitting
T
he recent TT Club seminar on Managing risk for tank
containers (see p 4) included a technical paper of great
relevance to repair depots.
Dr Ken Kirby, Ronald Veen and Arron Jackaman, of Brookes Bell
discussed Practical problems faced when tank containers experience
pitting.
They began by listing what the actual condition of a tank is likely
to be. It is often the case that there is limited information on
cleaning and maintenance programmes. Passivity and roughness
records are rarely kept if in existence at all.
In the event of damage to a container the easiest option is to
select the most recent cargo as the cause. However, it is sometimes
the case that the condition of the stainless steel within the tank was
poor, but the lack of information regarding the tank surface
condition means it is difficult to determine fault.
They then detailed the condition the tank container should be in
on delivery. The stainless steel needs to be clean, ie, free from visible
stains and dirt, and smooth, with an Ra value under 1.5mm.
But critically it also needs to be passive. As per the oxilyser3 test (a
stainless steel passivity tester), a value above 65, or preferably
higher, is required. Passivity is a property of the stainless steel to
resist corrosion under conditions where it should otherwise corrode.
A common misconception is that stainless steel is stainless, but this
is not true as stainless steel can be readily tainted by cargo residues
(resulting in a varnish). It can also suffer from rouging under certain
conditions. Another form of staining commonly observed is ‘tea-
staining’, which is caused by surface contamination.
Cleaning can be carried out by chemical means; but in extreme
cases chemical cleaning is not sufficient and mechanical cleaning
(polishing) becomes necessary.
The surface condition should be as smooth as possible to have a
positive effect on future cleaning regimes. Improvements in surface
roughness also enhance corrosion resistance.
delivered to the depot, an estimator visually accesses the condition
of the tank and may request an area of test grinding – this might be
supported by non-destructive testing (NDT) such as a dye penetrant.
If significant pitting is found the assessor may request for a small
area to be test ground to check physically how deep the pits are in
the test area. This is then used to help determine the likely time
required for repairs to the whole tank. Repairs are then conducted
by grinding and polishing and should be followed by pickling and
passivation.
Pitfalls
Within the pit aggressive ions accumulate combining with hydrogen forming acidic species
But the speakers said their experience is that surface roughness
condition is often ignored, because operators underestimate the
significance. In fact, surface roughness will deteriorate over time
and so should be monitored regularly. If the surface becomes rough
then the remedial action is to grind and polish (ie, buffing).
They then moved onto pickling and passivation. Pickling is the
removal of the metallic dirt and the damaged oxide (passive) layer.
This removes light discolouration and tarnish – particularly heat-tint
after welding – and also aids the promotion of beneficial elements
to the surface resulting in improvements to corrosion resistance.
Passivation is the formation of a new oxide layer by chemical
means.
So what happens in practice? When an off-hire tank container is
But there are pitfalls in the repair process. The estimator has no
realistic idea (even though experience will help to some extent) how
deep the pits are, and it is often the case that the test area does not
accurately represent the remainder of the tank.
It is also the case that some areas can be so badly pitted that
repair/remedial action is uneconomical – this won’t be known until
grinding happens and sometimes long in to the repair process. This
results in wasted costs for the operator.
Improvements can be made, however. For example, checking the
cleaning and repair processes by an expert, and also performing
stainless steel surface testing in regards to surface roughness,
ferrous contamination and passivity in order to prevent as much as
possible the pitting corrosion.
To get a correct understanding about pickling and passivation it is
necessary first to be aware that the difference between pickling and
passivation is like salt and sugar. For instance, is pickling required
after surface grinding and polishing, or will passivation only have a
better effect?
Brookes Bell has observed at tank repair depots that people are
referring to pickling and passivation, but in fact they only carried out
pickling without chemical passivation, while it should be chemically
passivated in order to reach its desired strong passive oxide layer to