Technical Presentations at the January 2014 Meeting

1.1  Materials for Diffusion-bonded Heat Exchangers’, Xiuqing Li, Heatric Division of Meggitt (UK) Ltd

Heatric designs and manufactures highly compact diffusion-bonded heat exchangers for use in gas cooling, gas dehydration and recently in floating natural gas liquefaction. They also have applications in power generation and chemical processing industries. Main benefits of the exchangers are space and weight savings.

One of the unique features of Heatric's heat exchangers is to join flat metal plates through diffusion bonding. Extensive work has been carried out on developing and qualifying robust process procedures for manufacturing this type of heat exchanger. Available materials include dual certified SS316/316L, SS304/304L, 22 Cr duplex stainless steel, 6Mo superaustenitic stainless steel, titanium grade 2, alloy 120 and alloy 617. More materials will be added to the list for the emerging markets with extreme environments.

This presentation focuses on the important factors affecting diffusion bonding. These include material properties, surface preparation, bonding parameters (pressure, temperature, time and atmosphere) and post bond heat treatment.

Developed diffusion bonding procedures must be fully qualified before they can be used in manufacturing heat exchangers. To get ASME U stamp certification, a series of tests shall be carried out strictly following the ASME code requirements. The aim of bonding procedure qualification is to confirm that developed procedures meet agreed requirements and ensure the bond quality will be maintained in the production.


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1.2 Keynote: ‘Corrosion Control inside Offshore Wind Farm Monopile Foundations’, Lisbeth Hilbert, FORCE Technology, Denmark


The design for corrosion inside a monopile foundation for offshore wind farms anticipates low, uniform corrosion rates in a closed compartment. Recent studies have revealed that in 2-10 year old foundations sea water and thus oxygen ingress occurs, increasing corrosion rates and localising attacks. Based on inspections and monitoring, the current corrosion state and prevailing mechanisms may be evaluated and form basis for future corrosion control strategies for existing and new projects.

Inside foundations, the risk related to corrosion is primarily changed and unpredictable service life expectancies, if reduced wall thickness values or corrosion fatigue cracking should alter the structural strength. Furthermore safety issues related to access to the inside of the foundations may occur. Different approaches may be taken to handle the unexpected higher corrosion rates. If the cause for increased corrosion rates is identified successfully, e.g. leaking seals at the j-tube inlet, remedial repair actions may be taken to reset the construction to a less corrosive condition. If corrosion rates are not alarming, another approach may be just to leave things as they are, and hope that corrosion rates will slow down. In order to make this approach acceptable from a safety issue, repeated inspections and/or corrosion monitoring will be needed.

Cathodic protection (CP) is already applied on the externally immersed surfaces of the transition piece (TP) and protects also the outside surfaces of the monopile (MP). Marine quality coatings additionally limit corrosion on the TP, but CP protects the steel if the coating should fail or be damaged. Likewise, as an alternative corrosion control strategy, cathodic protection can be applied inside the foundation. These surfaces are not coated, which requires a different anode design and distribution as well as it poses the problem that hydrogen generation may be substantial. The hydrogen gas must be vented from the foundation in order to avoid pressure build up, and the CP must be adjusted to avoid risk of hydrogen induced cracking, especially in sulphide rich sediment. This complicates the installation of inside CP. Another alternative control measure is to apply coating internally in the MP and TP’s or at least in selected zones considered to be mostly exposed to corrosion, e.g. at the prevailing water level. In practice, application is very difficult inside foundations offshore, and coating is not an easy remedial action for corroded structures. However, for smaller area and as a preventive measure, this approach seems to be promising.

Offshore wind farm foundations and sea current installations are also exposed to the marine mud zone. The long term experience on controlling corrosion in mud on oil & gas offshore platforms and large steel infrastructures is a good reference for identification of risks. The renewable energy production sites are however selected from other criteria e.g. strong winds, and structural designs are different. The closed compartment in monopiles differs from other known structures by containing stagnant water, which is often subjected to some replacement through leaking seals at j-tubes connections etc. This situation may cause corrosion in the mud zone that is more localised than experienced elsewhere. Marine mud is in general characterised by low oxygen content, large variations in resistivity, poor tendency to scale formation compared to sea water, and high bacterial activity facilitating microbiologically influenced corrosion (MIC), degradation of coatings, formation of iron sulphide corrosion products as well as increase hydrogen permeation and thus the risk of cracking. In spite of all this, long term experience shows that the uniform corrosion rate in the anoxic media may be negligible. Localised corrosion is, however, a risk especially for structures partially buried, where differential aeration cells may localise corrosion in the top part of the mud zone. Cathodic protection (CP) for corrosion control may therefore be applied, setting the protection potential in sulphide media minimum 100 mV lower than in sea water, but still avoiding too low potentials with major hydrogen permeation. Several authors have reported somewhat contradicting results on these issues, and failures due to insufficient CP relating to shielded part, disbonded coatings, and MIC have been reported.

No matter which approach is applied, hence forward, the conditions should be monitored to ensure that the new situation is withheld, and that the corrosion state of the inside surfaces is inspected at intervals. CP criteria, performance and coating condition should also be checked. An intelligent corrosion monitoring programme may enable thorough remote surveillance of the wind farm, but even coupons provide valuable documentation. Especially as offshore wind farms become larger, in cases with more than 100 foundations, the benefits of monitoring may prove substantial. With early warning capacity potential repair costs can be reduced significantly. [MCF members may contact the author for a pdf of the presentation - detals from Secretariat].


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4.1    High Strength Cupro Nickels’, Ivan Richardson, Copper Alloys Ltd


Copper Alloys Ltd, a marine wrought and continuous casting company based in Stoke on Trent, UK, employed the services of Mr Ivan Richardson, a metallurgical consultant, to investigate the possible enhancement of mechanical properties of key marine alloys. This resulted from continued pressure from design engineers to push the limits of properties, to enable Naval Platforms to last a life time without servicing.

Two alloys were examined which historically had performed well in sea water applications CuNi15Al3 and CuNi30Cr2 with documented corrosion rates of 0.02mm/year in pitting and crevice environments. By close examination of production parameters, control of chemistry and hot working techniques, considerable enhancement of properties were obtained:

CuNi15Al3 Din 2.1504


.

Size dia

0.2% proof Stress N/mm2

UTS N/mm2

Elong %

Hardness HB

Spec Min

15-50mm

640

780

10

230

Achieved

.

771

947

15

273

Spec Min

50-80mm

590

780

10

230

Achieved

.

785

903

12

260

High Strength

67mm

940

950

12

260

.

88mm

970

1012

6

283



CuNi30Cr2 Def Stan 02 824


.

Size dia

0.2% proof Stress N/mm2

UTS N/mm2

Elong %

Charpy Impact Strength Joules

Spec Min

.

300

480

18

NA

Achieved

135mm

594

699

18

137

.

130mm

633

727

19

109

.

125mm

721

782

14

111



Def Stan 02 824 is a cast specification and Copper Alloys Ltd achieved the above properties by hot working. This gives distinct advantages as regards the cast product with superior structural integrity as well as the mechanical test being taken from the forged product rather than a secondary cast test piece.

With the 125mm dia bar, although it technically falls outside the specification as regards elongation, the toughness remains high with a 111 Joules impact strength.

With over double the strength of the cast product, the wrought version gives the design engineer some distinct advantages.

In addition, with work conduct by the US Navy in 1979 it was found that CuNi30Cr2 could withstand sea water flow rates of 15metres/sec (50ft/sec) with acceptable erosion compared with Nickel Aluminium bronze which is restricted to 4.3metres/sec.

CuNi30Cr also performed 10 times better than 70/30 cupro nickel in sea water flow rate tests, conducted over a two and four year period.

Copper Alloys Ltd is also undertaking a one and two year corrosion programme to compare 26 different alloys which includes CuNi15Al3 and CuNi30Cr2 for pitting, crevice and galvanic corrosion.

The programme includes Nickel Aluminium Bronze, Curpo Nickel, Stainless steels, Monels, Super Duplex and Titanium as well as a range of other copper alloys and stainless coated samples.

This is believed to be the first time that such a extensive corrosion programme has been undertaken with specimens being tested under identical sea water conditions and will provide design engineers with a comprehensive data base.

An identical set of samples is also being suspended in neat sewage water in the inlet channel of a sewage treatment works where there are high levels of Hydrogen Sulphide.

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4.2   The Effect of Copper on Crevice Corrosion of Stainless Steels’, Roger Francis, RF Materials


Crevice corrosion leaks of superduplex sprinkler heads occurred in a firewater sprinkler system operating at 22°C. Tests showed that copper deposits from corroding copper alloys could act as very efficient cathodes and stimulate dissolution during the regular breakdown and repassivation of the passive film in crevices. Several other service failures have occurred due to a similar mechanism, which has not been previously reported. A paper on this subject will be submitted to Materials Performance later this year.

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