Technical Presentations at the April 2005 Meeting
types of very high strength copper nickel alloys are tested with respect
to environment sensitive mechanical properties which include hydrogen
embrittlement and exposure to sulphide and ammonium compounds.
It is found that Cu-Ni-Sn and Cu-Ni-Al-Mn-Nb alloys with nickel
content up to 25wt% are resistant to hydrogen embrittlement.
The Cu-Ni-Al-Mn-Nb alloys tested are also found to be resistant to
sulphide stress corrosion and stress corrosion in ammonium environments,
whereas Cu-Ni-Sn materials demonstrate susceptibility to stress corrosion
in these environments. A
study of factors controlling stress corrosion susceptibility of Cu-Ni-Al
and Cu-Ni-Al-Mn-Nb alloys shows the principal influences to be the degree
of age hardening, the grain size and the iron content. Thus, with necessary controls of the composition and
manufacturing processing of Cu-Ni-Al-Mn-Nb being undertaken, MARINEL 220
has been developed. In this
material, high mechanical strengths are achieved with the material’s
possessing a fine grain size and having resistance to stress corrosion. The use of the NACE TM-01-98-98 slow strain rate tensile test
is advocated as a production test method for very high strength copper
alloys to verify resistance to stress corrosion cracking susceptibility.
Register EMEA’s corrosion related activities encompass Asset Integrity
Management, Verification, Validation & Certification (Design
Appraisal) and Ad-Hoc Corrosion Consultancy.
To show the type of work undertaken, several case studies were
described in detail, including:
3.1 ‘Cathodic Protection of reinforced concrete in the marine environment’, Jim Preston (Corrosion Control Services Ltd)
Reinforced concrete is a material used universally and in the majority of situations it is a very durable material. Two main mechanisms can result of the corrosion of steel in concrete, these are carbonation (reduction in the alkalinity of concrete caused by acid rain) or chloride ingress through the concrete cover.
The main cause of deterioration is chloride ingress, and so marine structures are particularly at risk. Other common causes of chloride ingress are de-icing salts on bridges, wind blown chlorides or saline ground water. Many factors (such as concrete cover depth or porosity) may affect the time to corrosion. A case study was presented to demonstrate the extent and type of testing necessary when a structure is known to be at risk from corrosion.
CP is now commonly used by civil engineers to protect structures, either as a retro-fit as part of a repair project or sometimes (particularly in overseas markets) at time of construction. Different types of anode systems can be employed as part of an impressed current system and examples were shown using a MMO coated titanium mesh anode encapsulated in sprayed concrete, a MMO coated titanium ribbon anode installed both in slots in old concrete structures and cast into new concrete and a conductive coating used for inland systems. Latterly the technique has been adapted further, and an example of a steel framed masonry clad building was shown where CP can be used to protect the steel frame using discrete anodes.
CP of reinforced concrete is now a mature engineering
solution with supporting international codes.
Hydrogen induced stress cracking (HISC) issues
of weldable supermartensitic stainless steels has been experienced
by offshore operators in environments of H2S and under cathodic
protection. The conclusions
from a joint SINTEF/DNV/TWI study of this subject were:
Reduce polarisation to -0.800 V
(Ag/AgCl) - this gives no HISC for SMSS materials but the system could
not be qualified within the time limits available in the study
· Standard polarising to -1.050 V (Ag/AgCl) was qualified for butt welds for SMSS with:
o No fillet welds (eg those used for anode attachments) to SMSS
o The maximum allowable stress induced tensile strain to be 0.40 %.
o Only grades with 2 % Mo or above to be used.
heat treatment of 5 minutes
at 630ºC to be used
protection to new ISO Standard 15889-2 – Specific design details
· New design criteria have been developed. The following are the most important parameters that have been changed from previous standard:
maximum distance between anodes if adequate calculations are presented
reduced coating breakdown factors given for the new advanced coating
systems (multilayer PP)
positive protection levels allowed for CRA materials (eg – 500 mV for
concept for CP to insulated flowlines has been developed which includes
techniques for attachment of anodes to pipe
o Calculated voltage profile along the length of pipeline using finite element analysis
CP for the largest offshore pipeline project ever (Langeled and
Ormen Lange). Additional
CP requirements which have been agreed for these projects are:
For the current
densities - the upper current density curve as given in ISO
standard to be used
For critical and
strategic pipelines such as major trunk lines the total current demand
shall be multiplied with 1.5 as a safety factor
For these pipelines
the safety factor within the 1000m for pipelines connected to subsea
installations, platforms and landfalls shall be 3.
3. Pipeline heating issues
considerations for pipeline CP system requirements would arise if hating
of the pipe is necessary for hydrate control.
If direct electrical heating of the pipe is used, there would be a
need for AC corrosion control. Design
this system would be as follows:
allowable distance between anodes to avoid AC corrosion outside the
current transfer zone may be 4000 m for carbon steel and 5800 m for SMSS.
If the distance
exceeds 300m, the anodes shall be directly exposed to the seawater.
points between materials groups of different magnetic permeability shall
be defined and effects should be evaluated
points where diameter or wall thickness is changed shall be defined and
effects should be evaluated
All locations for
components shall be identified and anodes shall be installed so as to
maximise CP effectiveness and minimise AC corrosion
for AC corrosion in the current transfer zone shall be evaluated.