Technical Presentations at the January 2008 Meeting
talk focussed on two main topics; the behaviour of oilfield inhibitors on
preferential weld corrosion in pipelines carrying an oil and gas stream
and the effects of cathodic overprotection on welded structural steels
used for the latest generation of platforms and jack-ups.
current and self-corrosion rate measurements have been performed on the
weld metal, parent plate
and heat affected zones of welds sectioned from X65 pipeline steel and
tested in flowing brines saturated with carbon dioxide.
The effectiveness of inhibitors in controlling the corrosion rate
of each region of the weld has been investigated over a range of shear
stresses. The inhibitor
performance has been shown to differ in each region of the weldment and in
some circumstance the direction of the galvanic current has been found to
reverse so that accelerated localised corrosion occurred in the weld
metal. The roles of corrosion
product and inhibitor films were discussed.
in high strength steels are known to have an increased risk of failure by
hydrogen embrittlement, particularly when hydrogen sulphide is present and
when the welding process produces susceptible phases with high hardness. Hydrogen embrittlement has been studied in high strength
steels immersed in both natural and artificial seawater and held at a
range of cathodic protection potentials.
The effect of potential on the safe threshold stress intensity was
Barraclough gave a presentation on 'Welding, Joining and Corrosion'. He
gave examples of places where welding defects, shape,
microstructure and residual stresses cause corrosion, pitting,
corrosion fatigue, stress corrosion, erosion corrosion, bacterial
corrosion and galvanic corrosion.
Corrosion is a very complex process. It is often difficult to forecast rates of deterioration and impact on equipment performance and life.
Of all the mechanisms that can cause equipment to fail Corrosion is one of the probable but also the most preventable in Marine and Offshore industry
Despite the corrosion controls put in place failures still occur because the control systems themselves are vulnerable and susceptible to failure
In many cases the root causes of corrosion control failures are human and organisational rather than technical: Mistakes are often made for practical reasons without understanding the corrosion and reliability consequences
Reliability Engineers need to engage with Materials and Corrosion engineers to ensure that the right information gets to the right people at the right time
Employing experienced corrosion engineers in the organisation is necessary but not sufficient to prevent corrosion failures.
Key factors include:
and last but not least