Technical Presentations at the April 2008 Meeting

1.1  ‘Computer modelling of cathodic protection, applications on a harbour integrity design’, Ernesto Manuel Santana-Diaz, DNV and John Baynham, CM BEASY 

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:

    • A bare steel pile sheet with cathodic protection, nearby pile sheet perfectly coated
    • A bare steel pile sheet with cathodic protection, nearby pile sheet bare steel no protection
    • A ship berthed to the dock, hence interference effects when high, low tide and completed and uncompleted installation of anodes on the dock.

It 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.

3.1  Keynote 1:  ‘Stress Corrosion Behaviour of Nickel Aluminium Bronzes in Sea Water’, Jean-Loup Heuzé, DGA-DET-CEP/MC/PMA 7-9 rue des Mathurins 92 220 BAGNEUX,  jean-loup.heuze@dga.defense.gouv.fr 

Since 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,.. . . . . ...   . . 

NAB (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. 

Second, 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 pipes. 

Third, 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. 

At 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.

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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.

Conclusions were:

  • Hydrogen damage requires the activity of hydrogen in steel to exceed the susceptibility of hydrogen in a particular steel.
  • Susceptibility of steel is considerably enhanced by sources of tensile stress.  Environmental sources of stress, such as in deep sea service equipment, need careful consideration.
  • Hydrogen promoters, such as H2S, need be present at very low concentrations to enhance hydrogen activity.
  • The time scale for hydrogen equilibration in steel in seawater is up to a week for mild steel, but centuries otherwise, except for SCC. 

3.3    Keynote 2:  ‘Design and Fabrication Experiences with Titanium in Offshore Platforms’, Torfinn Havn, Ztrong AS, Norway 

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 ring main

– Sea water systems

– Heat exchangers

– Hypochlorite systems

– Ballast water systems

– Sea water pumps

– Desalination packages

– Drilling risers

Aspects discussed covered problems, fabrication (welding, forming), cost and availability. 

Conclusions:

1  By experience it is found that Ti gives the best material solution in fire water and sea water systems above 20°C

2  Ti need special focus on design and welding

3  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.

3.4  ‘Oil & Gas Related Corrosion’, Chris Fowler, Bodycote Testing 

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

follow standards, follow company procedures and design test protocols.  Various examples were then discussed to illustrate these points. 

Conclusions: 

  • Understand the material properties before embarking on a test programme
  • Understand the test methodology
  • Select the test method and conditions carefully

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