1. "Relating test procedures to service structures", Hector Campbell
The talk was about tests designed to reproduce, as far as possible, conditions of service of specific components of marine engineering equipment.In the May Jet Impingement test1, devised to test new condenser tube materials, recirculated natural sea water with air bubbles added was fed via a manifold to eight 2mm dia jets 2mm away from submerged specimens at 4m/s. The specimens were abraded strips of condenser tube or experimental alloy. It was later modified2 with new manifold allowing 20 specimens to be tested at 9m/s ± H2S additions.
The Campbell Condenser Tube Test3 was devised in 1970 for on-site comparative tests of candidate condenser tubes for a power station in Denmark where unexpected attack on 90/10 Cu/Ni had occurred. It tests ten 200 mm lengths of condenser tube with as-supplied internal surface finish under impingement, slow flow, heat transfer and crevice conditions. The apparatus was later used extensively by R Francis who set up units at Portland Harbour to study the effects, on standard condenser tube materials, of ferrous sulphate additions, chlorination and sulphide and ammonia contamination of the sea water.
In 1982 failures of alloy K500 bolts on cathodically protected riser pipes of a North Sea oil rig showed an outer band 1-2mm wide where the fracture was intergranular, the rest of the fracture surface being typical of ductile shear. The outer band contained 17av. cm3/100g hydrogen compared with >2 near the centre. The failures were attributed to hydrogen embrittlement (HE) but, since the alloy was at that time generally thought to be immune to HE, a test was devised to reproduce the known service conditions4. Four mm dia tensile test specimens were exposed under constant load conditions in cells through which deaerated sea water containing a small amount of H2S was passed slowly to waste, the specimens being potentiostatically controlled at -1.0V(SCE). Specimens removed and tensile tested after 60 days showed a big drop in both elongation and reduction of area. They had picked up large amounts of hydrogen and the fracture surfaces showed the same features as the failed bolts confirming that the bolts had failed by HE. The test has subsequently been used to investigate the HE resistance of other possible high strength undersea bolting material subject to cathodic protection and to establish for specific alloys the safe minimum potential, the maximum safe stress, the influence of overstrain and of metallurgical condition.
References:
1. R May, J Inst Metals, 1928, 40, 141
2. R May and R W De Vere Stacpoole, J Inst Metals, 1950, 77, 33
3. H S Campbell, 3rd Internat Conf on Marine Corrosion and Fouling, Washington, 1972. National Bureau of Standards
4. H S Campbell & R Francis, Brit Corros J, 1995, 50, 154
2. Corrosion of carbon-manganese steel welds, Trevor Gooch, TWI
Ordinary C-Mn and similar steels have fairly low corrosion resistance in most service media, and some form of corrosion protection is generally necessary. In consequence, any differences in corrosion behaviour at welds tend to be marked, and it is not always recognised that fusion welding can lead to significantly enhanced local attack in the weld area. However, preferential corrosion can occur under marine conditions in either or both of the heat affected zone (HAZ) and the weld metal of a welded joint. An outline was given of the present state of understanding of the problem, and of palliative measures.Attack on the HAZ in seawater has been encountered especially on hulls of icebreaker vessels, which may suffer loss of paint systems and of sacrificial anodes. The effect is particularly associated with the formation of hardened microstructures during welding, and hence may be largely overcome both by selecting base steels of low hardenability and by avoiding low heat input welding conditions with attendant rapid cooling.
Weld metal corrosion appears to stem primarily from galvanic action between the fused region and adjacent material, and is significantly affected by compositional differences. Available evidence indicates that the rate of attack is dependent primarily on Ni and Si contents, and the weld metal can be made cathodic to base steel by selecting consumables of increased Ni and reduced Si levels.
During electrochemical testing in seawater, measurements are typically made on the component parts of a weld (viz the base steels, heat affected zones and weld metal) to determine the relative rest potentials, the self-corrosion rate (LPRM) and coupling currents (ZRA). The possibility was pointed out of polarity reversal due to changes in surface condition, and the necessity was stressed of employing adequately long test durations.
3. Printed circuit heat exchangers in offshore applications, Tony Johnston, Heatric
Heatric's Printed Circuit Heat Exchangers (PCHEs) have pioneered the worldwide application of safe, robust, corrosion resistant, high pressure and temperature compact heat exchangers in hydrocarbon gas applications. They have opened new opportunities to innovative operators to cut costs through improved inherent safety and greatly reduced size and weight. On offshore platforms, savings commonly reach several times the price of the heat exchangers themselves when used project-wide.PCHEs have been applied as compressor aftercoolers and intercoolers, gas/gas exchangers, glycol and condensate exchangers, and multi-fluid exchangers, at cryogenic temperatures, pressures of hundreds of bar, and heat loads of many megawatts. Materials of construction include stainless steels and titanium
4. Comparison of high strength steel corrosion fatigue data with the revised PD6493 guidance
Alan Smith, TWI
1 “Recent
developments in the use of titanium offshore”, David Peacock (TIG)
An update and
review on some past applications was given, including: heat exchangers, with the
revolutionary new high pressure compact units now available from Rolls Laval;
piping systems particularly the all titanium fire water systems which are now
standard for safety weight reduction on Norwegian FPSO’s; propeller shafts to
overcome persistent crevice attack on steel shafts, and trials in hand to
optimise bearing materials; and lightweight titanium armour being fitted for
example to the French aircraft carrier Charles de Gaulle.
To the well appreciated
benefits of titanium, the ability for 100% recycling with a positive disposal
value at the end of component service, further reduces the life cycle cost of
titanium systems.
An updated MIR0 175
‘approved’ list was given which now includes Grades 29 and 28 (ruthenium
containing alloys). Ruthenium and palladium containing alloys will all meet the
most stringent classification within the NACE standard. Use of EFC 17 places a
full range of titanium alloys at the designer and users disposal. The recent
NORSOK restrictions on the service temperatures for duplex steel should result
in titanium being more used in higher temperature salt water service.
The generic high shock
resistance of titanium is a clear and considerable benefit not only for example
in existing fire water systems where damage and explosion tolerance is required,
but also for future naval vessels where some service specifications are calling
for survival of mine detonations immediately alongside the hull.
Concerns remain about
issues such as galvanic corrosion, hydrogen uptake, performance in
‘non-oxidising’ environments (TIG sheets covering these subjects were
available), and the latest TIG Design Guide titled the ‘Surface Finishing of
Titanium’ (available from 10 March) was introduced.
Phil Threadgill was
then introduced to comment on a new opportunity to review improved welding
methods for offshore risers in particular, and to complete much needed work on
subsea fatigue performance, particularly following plastic strain.
Several processes were
mentioned, including friction welding and its derivatives, keyhole plasma
welding and reduced pressure electron beam welding. In selecting a process, it
was necessary to take account of the maturity of the technology, availability of
equipment suppliers, robustness of the process (tolerance to small changes in
process parameters) and many other factors, and at present friction based
processes appeared to have an advantage.
Phil Threadgill announced that TWI were about to launch a Group Sponsored Project to investigate the topic of welding titanium alloy risers.
2 “Novel
techniques for the assessment of corroded pipework “,
Robert Gunn (Plant Integrity)
The
presentation covered three topics. The first described a new long-range
ultrasonic technique called TeletestTM, which can detect metal loss,
for example corrosion under insulation, in pipelines and process pipework. The
technique uses low frequency guided ultrasound capable of propagating over
considerable distances. Its long inspection range allows for extended lengths of
pipework to be examined without the need to remove large areas of insulation.
Second, automated
ultrasonic techniques for detailed corrosion mapping were covered. The
advantages and disadvantages of these techniques were described in relation to
an ultimate engineering critical assessment (ECA) of the component.
Third, an update was
given on standards for undertaking ECAs of corroded transmission lines, process
pipework and vessels. It was explained that both API 579 and BS 7910 were to be
finalised this year. These documents will offer engineers an enhanced scope to
pass corroded components as fit for purpose, but their application to corroded
welds and about nozzles required clarification.
3 “Coupling of
stainless steel and nickel aluminium bronze - successes and disasters”,
Roger Francis (Weir Materials)
Tests have been
performed at about 250C in both natural and chlorinated sea water on
couples of nickel aluminium bronze (NAB) and Zeron 100 superduplex stainless
steel. Two area ratios (1:1 and 1:10) were used in exposures of 60 days.
Under some circumstances severe localised corrosion of the NAB occurred. Recommendations are made for the safe use of nickel aluminium bronze valves in Zeron 100 sea water piping.
4 “Internal corrosion of seawater lift pump caissons”, Paul Badelek
(BP)
In
recent years the offshore oil and gas operators have experienced severe wall
thinning of carbon steel pump caissons. The internal corrosion of the uncoated
caisson is concentrated at the pump intake area. It is attributable to a
combination of galvanic corrosion by the submersible pump and its rising main,
and velocity enhanced corrosion. A theoretical model based on operational
experience has been developed by TOTAL which has highlighted the problem of
narrow annular gaps between the pump/motor assembly and the caisson. Good
correlation between actual corrosion rates and predicted have been found.
Accelerated corrosion
with perforations of pump caissons in only a few years has been the result of
the practice of internal coating the carbon steel caisson. At areas of coating
damage such as the centraliser locations, welds and pump intake area, wall
thinning together with fatigue cracking can result in total caisson failure.
Currently significant effort is being expended by the offshore industry to
inspect and repair pump caissons.
1. The development and service parameter characterisation of Nibron Special® - a very high strength cupronickel resistant to degradation in diverse marine environments, Keith Bendall, Columbia Metals Ltd
A very high strength cupronickel - Nibron Special - has been developed. The alloy design provides precipitation strengthening (proof strength up to over 700 N/mm -2), combined with high seawater corrosion resistance, complete freedom from hydrogen embrittlement and resistance to degradation in a wide variety of marine environments, such as ammonia and amine contaminated solutions.Detailed service parameter characterisation of the alloy by slow strain rate and C-ring testing will be described. This testing has been carried out to demonstrate the full resistance of Nibron Special to a range of potential operating regimes and has included hydrogen charged and sour environments, aluminium chloride at 80°C and seawater at 50°C.
To conclude some established and potential applications for Nibron Special will be reviewed.
2. Replacement of cadmium and chromates in metal finishing, Kevin Baldwin, DERA
The use of cadmium metal and chromate salts has become firmly entrenched in metal finishing over the past sixty years or so. Cadmium is usually employed where high corrosion resistance is required e.g. in the protection of non-corrosion resisting steels in marine environments. Cadmium-plated parts are invariably treated with chromate solutions that leave a thin, adherent, film on the surface often referred to as a "conversion coating" or passivation treatment. This film provides an attractive surface finish and additional corrosion protection as well as other desirable properties. Chromates, and other hexavalent chromium salts are also widely used in the surface treatment of other metals, including aluminium alloys, magnesium alloys and metal coatings other than cadmium (e.g. cadmium replacements). The pre-treatment of aluminium alloys is particularly dependent on the use of hexavalent chromium salts, for example, in chromic acid anodising and chromate filming treatments. In the aerospace industry, aluminium alloys are painted with primers that contain chromate salts as corrosion inhibitors. A particular advantage with chromates is their almost universal applicability and compatibility with a wide range of metals.
Cadmium and chromates have been used successfully for many years. The suppliers and end-users of these materials are familiar with their use and properties. They are also relatively cost effective. However, despite the many advantages of cadmium and chromates, they are highly toxic materials and the pressure to eliminate their use is now world-wide, hence replacements are urgently required. This paper will address some of the issues surrounding the replacement of cadmium and chromates in metal finishing and identify their main advantages and disadvantages. Recent work at DERA and elsewhere on alternatives to cadmium and chromates will be reviewed. The emphasis will be placed on the development of protective schemes that are completely free of toxic heavy metals. Both experimental and commercial systems will be discussed. Particular attention will be paid to work characterising the corrosion resistance of cadmium and chromate alternatives in marine and other chloride-bearing environments.
3. Mechanical and electrochemical interactions during erosion-corrosion in aqueous slurries
Anne Neville, Heriot Watt University
The use of engineering materials in aggressive applications associated with handling of aqueous slurries requires the ability to withstand excessive degradation due to the combined effects of mechanical erosion and electrochemical corrosion. It has been demonstrated, for a range of conditions, that there is significant interaction between erosion and corrosion processes. The extent of the interaction is a function of the material and of the environment (e.g. salinity, solids content, temperature etc.). Examination of material loss mechanisms under erosion-corrosion conditions enable a better understanding of how erosion and corrosion processes interact and help in the understanding of synergistic processes.
In this paper, a series of results from an extensive programme of research into the durability of high-grade stainless steels and related alloys, low grade steels, cast irons and surface engineering systems will be presented. The use of electrochemical techniques in the assessment of erosion-corrosion rates and mechanisms will be described and results of post-test assessment of failure mechanisms will be reported. The paper will focus on liquid erosion in the absence of solids and on erosion-corrosion in conditions of varying severity. The significance of corrosion in affecting the materials' erosion-corrosion behaviour and methods of avoiding excessive degradation in severe erosion-corrosion environments will be discussed.
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