1
‘Cost of corrosion: some preliminary findings for the offshore and
petrochemical sectors’, Nigel Whitehouse (Paint Research Association), Paul
McIntyre (Institute Of Materials)
The
cost of corrosion to the national economy, highlighted originally by the Hoar
report almost thirty years ago, is still a significant concern to industry in
this, the new Millenium.
As
part of a major suite of research projects [Degradation of Materials in
Aggressive Environments Programme (DME) 1998-200 1], the Department of Trade and
Industry (DTI) has awarded a project to Paint Research Association (PRA) to look
again at corrosion losses in a changing industrial landscape. The full title of
the project (DME 5.3) is Investigation of
the Cost of Corrosion, Risk Assessment Methodologies and Procedures for
Minimising Corrosion Costs.
PRA
is collaborating with the Institute of Materials (IoM) and the project is due to
be completed by the end of the year.
The
project has three objectives:
•
To estimate the cost of corrosion
•
To identify and disseminate procedures for minimising corrosion costs
through improved design, material selection and protective coatings
•
To establish the risk assessment methodologies which make best use of
resources
Information
is being gathered primarily from the following five industry sectors:
• Chemical/petrochemical
production and associated plant and equipment
•
The food and drink industry and associated plant and equipment
•
Offshore fabrications and equipment supplied by vendors
•
Constructional steelwork
•
Automotive OEM manufacture and associated components
The
sectoral approach ensures relevance and focus in a field encompassing diverse
corrosive environments. Industrial
involvment in the project is in place. An Industrial Advisory Group (JAG),
chaired by Dr J A (Tony) Richardson of ICI Engineering Technology, has been
established and is providing guidance, advice and industrial experience.
Sectoral subgroups are also being established.
The
findings from this study-which are likely to set the corrosion control
benchmarks for the first decade of the new Millenium- will be embodied in a General
Handbook and Five
Practical Sectoral Guides. Each guide will highlight specific control
strategies and good practice. PRA
and IoM welcome all expressions of interest in the project and relevant data.
2 ‘A review of procedures for assessing corroded vessels and pipelines’, Alan Smith (TWJ)
The talk summarised the assessment
strategies given in ASME B31G, BS79 10, DNV
RP-F 101 and API RP 579
for considering corroded
pressurised components.
The methods were
compared against a database of burst test results and strengths and weaknesses
of the different methods were discussed.
3
‘Nickel alloy in seawater service’, Steve McCoy
(Special Metals)
The
excellent corrosion resistance of nickel-alloys has been put to good use in
marine engineering for many years. Some applications, such as oil field
equipment and bolting, require high levels of strength as well as corrosion
resistance. The mechanical properties and corrosion resistance of high-strength
nickel-based Corrosion Resistant Alloy ( CRA’s
) are
discussed. New high strength nickel-alloys and their weldments exhibit excellent
resistance to sour oil well environments, hydrogen embrittlement and seawater
corrosion. Solid solution nickel-based alloys such as INCONEL alloy 686 obtain
their strength through cold work, while other highly corrosion resistant
nickel-alloys such as INCOLOY alloy 925 and INCONEL alloy 725
are precipitation hardened.
Both the cold worked and the precipitation hardened alloys exhibit exceptional
strength, ductility and toughness.
4
‘Alloy 825 and 718 gasket corrosion in deep water (500 m) connections,
Chris Amon, Amon Metallurgical Engineers Ltd
Corrosion
has been observed on a number of alloy 718 (UNS NO7718) and alloy 825 (UNS
N08825) gaskets from subsea connections. The gaskets were in contact with either
weld overlaid alloy 625 (UNS N06625) or super duplex stainless steel hubs (UNS
S32750).
A
few hubs also showed some limited corrosion in regions where severe gasket
corrosion has occurred. Corrosion was more prevalent on jumpers that had been
exposed to sea water for a few months at alloy 718 (UNS N077 18) gaskets in
contact with alloy 625 (UNS NO6625) weld overlay.
General
observations were that short term exposures of up to 20 days did not result in
corrosion but longer exposures of 40-50 days resulted in severe attack. A
programme of work was carried out to determine the reason for the observed
attack and to suggest ways of avoiding it in the future. The findings of the
study are described in a paper presented at NACE 1999 corrosion conference in
San Antonio Texas.
1
‘Performance of a highly alloyed stainless steel in seawater cooled
plate heat exchangers’, Lena Wegrelius (Avesta Sheffield)
So
far stainless steels have not been resistant enough to be used in plate heat
exchangers in continuously chlorinated ocean water. However, the second
generation’s superaustenitic steel Avesta Sheffield 654 SMO®
which has an outstanding resistance to pitting and crevice corrosion, was
believed to be very suitable for this application. A real plate heat exchanger,
equipped with 654 SMO® plates,
has therefore been tested with North Sea water heated to different, elevated
temperatures. The severity of the tests has been increased by continuously
chlorinating the water to 2 ppm of residual chlorine. The paper describes the
results of the tests and discusses the application possibilities of 654
SMO®
in comparison with those of titanium and Ni-Cr-Mo alloys.
2
‘A model for corrosion of the depleted zones around sigma precipitates
produced during welding of super duplex stainless steel’, Roger Francis (Weir
Materials)
Superduplex
stainless steel has now been in service since 1989, with well over 1 million
welds, and generally few problems have occurred. However, during welding,
particularly of small-diameter thin-walled pipe, it is possible for small
amounts of sigma phase to be precipitated in the low-temperature heat-affected
zone. The depletion of chromium and molybdenum around the precipitates reduces
the local corrosion resistance, and the question that is frequently asked is:
how much sigma phase can be tolerated before corrosion resistance is
significantly impaired?
In
the paper a simple model is presented for the depleted zone around sigma
precipitates produced during the welding of Zeron 100 superduplex stainless
steel. The requirements for corrosion to propagate are evaluated, and the
precipitate size is shown to be the most critical factor. The tolerable level of
sigma phase is shown to vary hugely with particle diameter.
Using
typical precipitate sizes found in Zeron 100, the model’s predictions are
shown to correlate well with the results of corrosion tests of welds in
chlorinated seawater. Similar
results are found for sour oil and gas process brines and flue gas
desulphurisation slurries.
The
results of the modelling exercise have important implications for engineering
fabrication of superduplex stainless steels. Because of the dramatic influence
of precipitate size there is no single figure for the tolerable level of sigma
phase which is acceptable. It is concluded that fitness for purpose tests for
welds are more relevant than metallography and counting of sigma particles.
3
'Metallurgical examination during weld procedure qualification for
ferrite-austenite stainless steels’, Trevor Gooch (TWI)
The
thermal cycle experienced in a fusion welding operation can significantly modify
the structure of wrought duplex and superduplex stainless steels, leading to
reduction in service properties of the weldment relative to the base material.
Accordingly, testing is necessary to qualify a welding procedure for a given
application. This normally involves assessment of joint soundness, of mechanical
characteristics, and of corrosion resistance. Because properties are highly
dependent on the weld area microstructure it has become common practice for weld
procedure qualification (WPQ) testing of ferritic/austenitic steels to include
also metallographic evaluation. This entails demonstration that the
ferrite/austenite balance is within set limits together with little or no
intermetallic phase formation, although other microstructural changes such as
nitride formation are not generally recognised.
However,
the imposition of microstructural evaluation leads to problems in qualifying a
welding procedure, not only because quantitative limits may vary appreciably
between different specifications but also because the criteria may be
conservatively and unrealistically set. Moreover, quantitative metallography
requires considerable experience and skill, and is subject to inherent
statistical variation, so that debate arises regarding the results obtained.
In the
presentation the philosophy of incorporating quantitative metallography into WPQ
testing are discussed. Limits of accuracy of measurement are indicated. In
particular, it is pointed out that service properties are not directly
relateable to the metallographic criteria generally imposed; chloride pitting
resistance, for example, depends not on the intermetallic volume fraction at
levels of practical concern but rather on the temperature of formation of the
intermetallic constituent and the particle size. It is concluded that acceptance
of a given weld procedure should be based primarily on measurement of the
mechanical and corrosion properties of practical concern and that
microstructural evaluation should not be mandatory. Metallographic investigation
remains of potential value should service entail characteristics not evaluated
in WPQ testing, and to indicate the likely tolerance of the welding procedure to
variation in conditions used.
1 'Localised Corrosion: Implications and assessment methods for structural integrity' Bob Akid (Sheffield Hallam University)
The initial development and growth of defects from 'engineered' surfaces, i.e. fine abraded, polished, shot peened etc., often dominates the resulting component lifetime, particularly for materials of high strength and limited ductility. When subject to the conjoint action of stress and environment this lifetime is impaired and reductions in fatigue strength are often observed resulting from a reduction in defect development time, often termed 'initiation', and enhancement in defect growth rate.A number of factors exist which influence the rate at which defects, such as pits/cracks, develop[1]. Included in these are; physical and chemical material surface condition, the nature of the loading mode, test frequency and electrochemical micro-climate at the metal/solution interface. Based upon corrosion experiments conducted under cyclic and static stress, using low and high strength steels and stainless steels in chloride environments, the following events; surface film breakdown, pit development and growth, pit/crack transition and environment-assisted stage I and stage II crack growth have been observed. Included in these experiments is that of the Scanning Reference Electrode[2] a technique adapted to evaluate stress-assisted localised corrosion, a process considered to be of primary importance during the early stages of stress corrosion and corrosion fatigue cracking; particularly for actively corroding systems.
The presentation will provide a general background of the implications of localised corrosion on structural integrity. Attention will be paid to the synergistic effects of stress and corrosion and the importance of pitting and the pit/crack transition on component corrosion fatigue lifetime. In addition the potential role of the new scanning electrochemical techniques for the evaluation of the early stages of stress corrosion and corrosion fatigue cracking will be discussed.
References;
1. R Akid (1999)
Influence of Environmental and Loading Variables on Corrosion Fatigue Short Crack Growth. Proceedings of the International Conference on Environmental Degradation of Engineering Materials, Gdansk-Jurata, Poland. Vol.1, pp 113-124
2. R Akid (1995)
Localised Corrosion - A New Evaluation Approach
Materials World, Vol 3, No. 10, pp 522-525 (Published by The Institute of Materials)
2 'Monitoring of polyurethane coatings in flowing saline slurries' Rob Wood (Southampton University)
This paper presents the degradation mechanisms of porous and non-porous PU coatings on steel subjected to slurry erosion-corrosion using a purpose-built apparatus. The samples were subjected to a slurry jet made of 3.5% wt. NaCl solution, containing 0-3% wt. sand particles (90° impact angle, jet velocity 4m/s). Prior to testing the corrosion reactions present on the steel substrate were examined by Electrochemical Impedance Spectroscopy (EIS) and during the test by Electrochemical Noise Analysis (ENA) on the corrosion current. The non-porous coatings remained fully protective for at least 10 months when exposed to a purely corrosive solution, and until total stripping by the sand particles when exposed to a corrosive slurry. In contrast, for the porous coatings, corrosion reactions were rapidly initiated on the substrate after solution transport, via defects in the coating, to the coating/metal interface. However, no synergism was detected between the erosion and the corrosion under these test conditions. Analysis of corrosion current fluctuations provided information on the coating degradation mechanisms. The mean and standard deviation were found to act as effective qualitative indicators of the extent of coating damage. The skewness, kurtosis and PSD slope indicated the initiations of the corrosion reactions on the substrate, i.e. the beginning of coating breakdown.3 'Hydrogen embrittlement in superduplex stainless steels', David Baxter (DERA)
Duplex and super duplex stainless steels are materials that offer an attractive combination of high strength, toughness and corrosion resistance. As a result they have found widespread use in the offshore oil and gas industry. However, as these alloys contain nominally 50% ferrite there is potentially a risk of hydrogen embrittlement if critical combinations of hydrogen content and mechanical stresses and strains are exceeded. The problem lies in identifying the detailed nature of these critical criteria for embrittlement.In many cases the use of duplex alloys is accompanied by the use of a cathodic protection system. Cathodic polarisation may occur either inadvertently via galvanic coupling to a carbon steel, or by design if the material is to be used in severe environments where localised corrosion may be a problem. Until relatively recently, service experience with these materials had suggested that the performance of duplex stainless steels under cathodic polarisation was not of particular concern, and problems were thought to be restricted to cases where poor welding or processing had resulted in the formation of excessively high ferrite contents. Laboratory tests indicated that cracking only occurred at very low polarisation potentials and very high applied stresses, well above the proof stress and typical design stresses. However recent experience suggests that microstructurally sound material can also be sensitive to hydrogen under certain conditions. There is a desire therefore to more precisely define the environmental and mechanical conditions under which this form of embrittlement may be a problem. This presentation describes a program of work dedicated to investigating these issues. In particular it explores the effect that stress concentrators and/or cyclic loading can have on the performance of these alloys under cathodically polarised conditions.
Constant load tensile testing has shown that, in agreement with other work, very high stresses are required before specimen failure due to hydrogen embrittlement takes place. The introduction of a notch, despite leading to premature formation of small surface cracks, did not affect the overall resistance of the material in this type of test. This can be attributed to the restraining action of the austenite phase which restricts the extent to which cleavage cracks in the ferrite can propagate. Cyclic loading of unnotched specimens has similarly been shown to have little or no effect on the materials resistance to embrittlement. However, during fatigue crack growth testing the effects of cyclic loading and stress concentration are combined to produce a region of localised cyclic plasticity, and under these conditions a dramatic effect of cathodic polarisation is observed. In these circumstances the degree of protection offered by the austenitic phase is reduced, as mechanical processes will soon lead to crack advancement through this phase, and hydrogen enhanced crack growth through the ferrite phase can then continue. The availability of suitable fatigue data is of course vital to allow a realistic component lifing calculation. Unfortunately obtaining representative data is further complicated by the fact that rates of hydrogen diffusion in these materials are low, and growth rates are strongly frequency dependent. Considerable care is therefore required when interpreting the results of accelerated laboratory testing. Low frequency testing has shown that the embrittlement process promotes a Kmax controlled cyclic cleavage process. At high stress intensities growth rates become significantly higher than those seen in air. Further work examining the effect of various microstructural parameters on fatigue crack growth behaviour is currently underway.
4 'Corrosion performance of titanium heat exchangers', John Fowler (Rolls Royce Plc)
Some general corrosion behaviour of titanium in marine applications was discussed. Localised corrosion mechanisms such as crevice and pitting were addressed and the excellent resistance of titanium alloys to a wide range of chemical environments was highlighted. The issue of galvanic corrosion and methods to reduce detrimental effects were discussed also. Details of a case study of a titanium wellhead cooler in North Sea operation were presented.