CORROSION IN BALLAST TANKS – IS RUST A MUST?
M R Kattan, Safinah Ltd, UK
SUMMARY
This paper considers the impact of ch anges in design, materials, production technology and regulations over the last 10-
15 years on the corrosion problems in seawater ballast tanks of large bulk carriers (tanker and dry bulk). It reviews the
challenges that have arisen and makes recommendations as to how these could be met by b etter design, p lanning and
control to properly integrate the coating process into the lifecycle of a ship:
1. IN TRODUCTION
As long as ships hulls are designed and built
predominantly of mild steel and set afloat in seawater,
then corrosion will eventu ally tak e place despite the best
endeavours of yards, owners and paint manufacturers to
prevent or delay its inception.
That is because the 4 elements needed for corrosion are
usually present:
•
Air (oxygen)
•
Cathode
•
Metal
•
Electrolyte
(Usually remembered by the acronym – ACME).
The purpose of coatings is to create a barrier b etw een the
Metal and the Electrolyte thus preventing corrosion. In
seaw ater ballast tanks (ballast tanks) this barrier is
usually a physical one created by the adhesion of the
coating to a prepared surface and the creation of a
uniform paint film. Thus, the integrity of the coating film
and its adhesion are p aramount to its p erformance as a
barrier against corrosion.
The integrity and adhesion are influen ced by a numb er of
factors some of which are:
•
Design of the space
•
Material selection
•
Production processes (including re-work)
•
Surface preparation
•
Application (method and timing)
•
In service conditions
•
Inspection
•
Coating chemistry
All of these to a greater or lesser extent play a key role in
the coating performance but design and material
selection h ave far and away the larg est influence.
1.1 SOURCES OF CORROSION
Corrosion of ships is a result of several different typ es of
mechanisms:
•
General corrosion typically at a rate of 0.1mm per
year in terms of wall thinning [1]
•
Galvanic corrosion between two metals with
dissimilar electrochemical properties (this can be
induced by a change in the chemical composition of
mild steel between one part of a steel p late and
another, so should not only be viewed as dissimilar
metals but also the same metal with a different
potential also). This is th e most common mechanism
on ships [2]
•
Salt spray and atmospheric corrosion
•
Chemical attack
•
Microbio logical Influenced Corrosion (MIC – from
mud accumulations), such as Sulphur Reducing
bacteria (SRB) and also anaerobic bacteria. These
microbes can cause localised change in the
environment, which can promote aggressive pitting
and other types of corrosion.
Thus although the mechanisms of corrosion are well
understood, there is still a problem in properly managing
the protection of the vessel against corrosion. For ballast
tanks all of the above mechanisms are possible w ith the
excep tion of Chemical attack, in normal service
conditions. However even this may change with the
advent of ballast water treatmen t systems as required
under IMO regulations [3,4,5,6].
2 . REGU LATORY ENVIRONMENT
Despite numerous regulations, [7,8,9,10 and most
recently 11] guidelines and considerable technological
advances in coating technology and shipbuilding and
repair technology, many recurring problems arise in
ballast tank coatings failure. These gen erally manifest
themselves in failures such as cracking, disbondment,
blistering, etc. these are caused by a mix of poor design,
poor specification, poor production control and poor
maintenance
The question that must be asked therefore is: Is the
problem being addressed and tackled in the correct
manner?
In fact over th e last 15 years no fewer than 12 different
regulations, standards or gu idelines have been
published/issued world wide in an attempt to tackle this
relatively in transigent problem. These include work by
