Technical Presentations at the April 2008 Meeting
This presentation demonstrated with some 3D mathematical modelling how the cathodic protection systems installed on a dock and on a ship can interfere in several conditions. The base case was instigated by some Aberdeen Harbour engineers, who considered that the installation of some anodes in sheet pile surface of the harbour and leaving the nearby surface without anodes could affect the integrity of the harbour and the anodes installed. This was confirmed with the use of computer modelling.
Several cases were studied:
was concluded that a ship berthed to the dock could have a detrimental,
prejudicial effect on the hull and dock anodes and the dock
structure near the propeller together with a beneficial effect in areas
away from it amongst other effects. These effects should be considered in
the initial installation of the anodes as they could affect the integrity
of the hull and the dock in the long term.
about 1960, French Navy use Nickel Aluminium Bronzes (NAB) for numerous
marine applications: cast propellers, wrought high pressure piping
systems, flanges and valves, heat exchangers,.. . . . . ...
(Cu Al9 Ni3 Fe2) is used in high pressure piping systems due to their high
mechanical characteristics associated to a good corrosion resistance in
sea water. Until then,
susceptibility of NAB to stress corrosion cracking in sea water was not
reported. However, over the
last 12 years, CuAl9Ni3Fe2 has exhibited some cases where cracks were
found on piping systems located in sea water.
First, a preliminary failure analysis was carried out by "DCNS/CETEC
- Cherbourg" on cracked samples taken from a pipe after more than 20
years of exposure in seawater. In
order to gain insight to the mechanisms by which service damage develops,
we investigated the environmental and metallurgical conditions under which
stress corrosion cracking of CuAl9Ni3Fe2 occurs.
numerous tests using U-bend and C-ring specimens were conducted by DCNS/CETEC,
in various environments and conditions.
Typical cracking obtain with these tests was compared to these
detected on pipes. The
influence of possible pollution such as ammonia has been found
predominant. These test parameters have been determined to simulate SCC on
Nickel Aluminium Bronzes. The
cracks obtained in these tests are similar to those observed on actual
a specific study to understand the mechanisms of SCC on NAB was conduct by
"ENS Mines de Saint-Etienne Materials Science and Structures
Laboratory" for a PhD thesis. Slow
Strain Rate Tensile tests (SSRT) were conducted in synthetic sea water
with different pH (acid and alkaline solutions) at free potential and at
-1050 m V/SCE. Influence of
microstructure, strengthening, pH, cathodic protection were investigated.
free potential, with acid pH < 4, we don't observe a SCC mechanism but
a general intergranular corrosion occurs.
With alkaline pH, intergranular cracking always occurs but
transgranular cracking increases. Cracking
proceeds by dissolution of the more anodic phase and fracture mode depends
on phases nature and repartition. When
the second phase is martensitic, back-scattered SEM observations show that
dealloying of the martensite is associated to the crack growth. At pH 10 or in synthetic sea water, we also observe local
transgranular cracking of the phase.
The SCC phenomenon probably initiates during the dry-docking period in the presence of stagnant sea water and in the absence of cathodic protection. Propagation occurs during operation under cathodic protection (-1050 mV/SCE) and stresses of in services conditions. Influence of hydrogen is excluded.3.2 ‘Factors Informing the Hydrogen Induced Damage to Steels in Upstream Service', Frank Dean, Ion Science, Frank.Dean@ionscience.com
This paper looked at hydrogen chemistry, examples of hydrogen damage, linking hydrogen sources and susceptibility, steel qualities causing hydrogen susceptibility, and hydrogen charging.
This presentation looked at experiences with Titanium installed offshore Norway. The motivation for use of Ti was that considerable corrosion problems are experienced in sea water systems with carbon steel, copper alloys and stainless steels.
Services where Ti is used:
– Fire water
systems (piping, valves, deluge, sprinkler)
– Fire water
– Sea water
– Sea water
discussed covered problems, fabrication
(welding, forming), cost and availability.
By experience it is found that Ti gives the best material
solution in fire water and sea water systems above 20°C
Ti need special focus on design and welding
The availability of the material is limited and engineering has to
be aware of the situation
4 Care is to be taken when Ti is to be coupled to other materials.
This presentation looked at the reasons for undertaking Sour Service Testing, which could be: to qualify a new material; qualify a new process e.g. weld procedure; quality assure the manufacturing route or to avoid failures. It then considered how to test: looking to
standards, follow company procedures and design test protocols.
Various examples were then discussed to illustrate these points.