1.
"MIC" - Dr Andrew M Pritchard, AEA Technology
The role of single organisms pertinent to corrosion was discussed. Sulphide Oxidising Bacteria (SOB) may produce local pH <1 due to H2S04 production or even result in elemental sulphur. Sulphate reducing bacteria (SRB) depolarise cathodic reaction although hydrogenase alone is not sufficient for the observed rates. With SRB and oxygen present, rates will increase further. Slimeformers consume oxygen (50μm thick results in anaerobic environment), trap debris and provide nutrients for other organisms whilst iron oxidizers can cause localised corrosion. Mechanisms are mostly unproven at present. Methods of detection in-situ/on-site were described and followed by a series of examples involving steel, stainless steel and cupro-nickels.
2.
"Corrosion of duplex stainless steel", Mr Robert N Gunn, TWI
A
multi-sponsored research project was reported investigating the corrosion
behaviour of a range of 22 and 25Cr duplex stainless steels. Studies
involved weldments, both with and without crevice formers, in exposure trials,
potentiodynamic anodic polarisation to determine the critical pitting
temperature and a limited range of studies in H2S media.
Welding consumables were varied from the manufacturer's recommendation,
alternative consumables to alloy 625. The results of these studies were
explained by means of the Pitting Resistance Equivalent (PRE) based upon the
composition of the material (parent or weld metal). Variants of the PRE
equation were discussed noting influence of Nitrogen (PREN) and tungsten (PREW).
3.
"Corrosion behaviour of martensitic stainless steels in slightly
sour oilfield environments", Dr Mick Schofield, Cortest Laboratories Ltd
The
generally accepted test method for assessment of susceptibility of material to
sulphide stress corrosion cracking is NACB TM0177, although due to the lack of
correlation to service environments variants of the test are frequently
used. Significant changes are being incorporated in the 1995 version
regarding test environment, materials etc. Data were presented regarding
performance of 13Cr, super 13Cr and 15Cr martensitic stainless steels.
(Note a compilation of available data can be obtained from the author).
Electrochemical potentiodynamic polarization scans were shown to vary with
exposure period and the impact upon such tests was discussed.
4.
"Galvanic coupling of materials with stainless steels", Dr
Roger Francis, Weir Materials Ltd
The
electrochemical behaviour of stainless steels under different conditions in
seawater and cathodic efficiency of such materials were described.
Problems were often associated with coupling dissimilar metals in gaskets etc,
although the environment may sometimes play the major role in behaviour e.g.:
influence of chlorination. In general the passive metals are
compatible. Service problems were then described noting possible variation
in satisfactory crevice behaviour of Zeron 100 compared with other 25Cr duplex
stainless steels could reflect the acidic local service environment.
Conclusions were that materials selection can still be problematic, often this
related to minor components in a system, but failures resulted from errors which
could have been avoided.
1.
‘Corrosion Design Data Requirements – a design house view’,
Dr Mike Swidzinski (Kvaerner H&G Offshore)
Materials
selection and corrosion control play a fundamental role in the determination of
overall project economics and ultimate concept feasibility.
In
most projects the development of a Materials Selection Philosophy (and possibly
Corrosion Control Strategy) usually takes place during the early stages of
design. The conclusions drawn and
reliability of recommendations made are only as good as the accuracy and quality
of the initial data supplied.
The
importance of getting the materials selection philosophy
"Right-First-Time” is, therefore, clear.
To achieve this requires the provision of accurate and reliable corrosion
design data.
This
presentation considered the broad spectrum of materials and corrosion
engineering design data issues - ranging from the early definition of corrosion
design data requirements by the design house through to examining the client’s
needs and the impact and consequences of design changes at later stages in a
project's maturity.
Materials
and Corrosion engineering design issues addressed included:
-
Corrosion Design Data definition
-
Project Design Philosophy - Design House Criteria
-
Prospect Development Philosophy —Client Criteria
-
Corrosion Modelling Approaches
-
Cost Benefit Analysis and Life Cycle Costing (LCC)
-
Application of New Technology & New Ideas
The
CEG is a computer program developed around a mathematical model of crevice
corrosion of stainless steels m aqueous chloride environments.
It is a predictive tool aimed at designers and engineers to help them
select the appropriate stainless steel for use in a given water.
As such it is applicable to many industries, including marine,
desalination, power and water, pulp and paper and so on.
The
model is based on the four stage process of crevice corrosion.
Firstly deoxygenation of the solution takes place as a result of the
passive current and oxygen reduction reactions occurring within the crevice.
This is followed by the passive current flowing within the crevice, and the
cathodic oxygen reduction occurring outside the crevice.
This brings about a build-up of metal ions in the crevice, in particular
Cr, which hydrolyses and causes the pH to fall.
At the same time electrical neutrality is maintained by a build up of
anions within the crevice (chloride and sulphate ions).
Depending on circumstances some back diffusion of species out of the
crevice can occur. The
aggressiveness of this solution increases until either the protective passive
film on the stainless steel breaks down resulting in corrosion, or, it reaches a
steady state value (due to a balancing of migration into the crevice and
diffusion out) in which case the passive film will not break down and no
corrosion occurs.
The
CEG is constructed in a user friendly form, with much information and many help
statements available in all parts of the program. The major inputs to the model relate to alloy selection,
alloy application, the aqueous environment and the form/condition/surface finish
of the material. Each of these
exist as a separate menu in the program through which the required parameters
are input. For the experienced user
there is a rapid input menu item which allows easy input of everything except
the alloy selection.
3.
‘Service Experience of Marinel High Strength Cupronickel Alloy’, Dr Clive D
S Tuck (Langley Alloys)
Marinel
was developed from Hiduron l9l, which is an age-hardenable Cu-Ni alloy,
Marinel
was shown to: have good hydrogen embrittlement (HE) resistance in tests at BNF
(1989-91). The alloy will absorb
hydrogen generally but no HE results. Galling
resistance was noted to be very good due to the copper content.
Information
was then presented on service performance, varying from fasteners for duplex
stainless steel valve installations under cathodic protection to topside
applications. One failure was
reported which was attributed to the use of thread lubricants (containing
molybdenum disulphide, trichloromethane, aluminium powder) on a DSS pipeline at
elevated temperature (7OoC). It
was postulated that H2S (or precursor) was formed from the reaction of
MoS2, and HCl, the latter produced due to the reaction of the Al with
HCCl3. Cracking resulted
from over-tightening and the presence of the aggressive sulphide species.
A
service performance indicated no galling to occur with Marinel, the use of
thread lubricants was not recommended for use with Marinel.
4.
‘Materials and corrosion experiences of British Gas’, Mr Richard
Knight, British Gas (ERS)
As
a major operator in the production and use of hydrocarbons, British Gas has
experienced a wide range of problems in the use of materials to combat
operational conditions. Some idea
of the variety and extent of these problems has been indicated together with
more detailed individual examples. The
growth of the industry has led to more emphasis being placed on financial,
environmental and safety benefits that result from detailed investigation of
individual problems.
The presentation ranged from corrosion associated with the storage of LNG to problems in offshore process equipment.
1.
‘Materials and Material Selection in the 0il and Gas Industry’, Mr
Roger Hayes (John Brown Ltd)
Current
materials and material selection problems in the development of offshore oil and
gas field developments were discussed. Examples
of materials selection and design considerations involved were given for some of
the main components of such oil/gas field developments, including pipelines,
jackets and topsides processing equipment.
Topics covered included the use of structural steel castings, clad steel
vessels and piping, duplex stainless and other corrosion resistant alloys for
service in CO2 containing process fluids, and corrosion problems in
sour service and sweetening systems.
2.
‘Extending the Applications of Titanium to Marine Environments’, Dr
Trevor J Glover (Titanium Information Group)
The
use of titanium in power station condensers and for plate and tube/shell heat
exchangers on offshore platforms is well established. The excellent service experience, in some cases more than 30
years, has given the process industry the confidence to specify the material for
an ever increasing number of components to handle sea water.
The
presentation focused on these new applications such as high pressure drilling
risers, fire water systems, valves, umbilicals and large diameter pipes. Whilst
corrosion resistance is the prime motive in using titanium in marine
environments its low density, high strength and competitive life cycle cost are
also key factors to be taken into account.
3.
‘Mapping of Corrosion Using T-Scan Ultrasonic Imaging’, Mr Bryan
Kenzie (TWI)
The
aim of this presentation was to share the experiences of TWI's NDT department in
the field of application of ultrasonic T-scan systems for corrosion mapping. Typical
applications such as internal corrosion in ships hulls, offshore structures,
pipes and vessels were discussed, together with the range of potential
inspection solutions including traditional point or grid measurements using
A-scan flaw detectors or digital thickness meters up to advanced computerised
T-scan systems.
T-scan equipment, techniques and capabilities were covered in detail together with a range of corrosion map images acquired in the field. The talk finished with examples of T-scan images that had been processed by software recently developed at TWI which automatically removes plate flaws such as laminations and inclusions thus making subsequent analysis easier.
1. 'Hydrogen
Embrittlement of Corrosion Resistant Alloys', Dr Roger Francis, Weir Materials.
Early
failures due to HE of Monel K500 under cathodic protection led to concerns over the
possibility of HE of other CRA's. Studies of stressed and unstressed K500 showed
both to embrittle. Further failures of other CRA's raised concerns over their
suitability, particularly as fasteners when CP was applied. Work was presented
showing that the HE cracks originated in the ferrite phase but stopped at or
passed around the austenite. However, only severe (SSRT) conditions of high
plastic deformation showed any reduction in elongation for more recent alloys of
50/50 ferrite/austenite phase balance: Zeron
100.
Service
failures have been restricted to heavily cold worked tubulars and no failures have
occurred under operating conditions. Provided the yield strength of highly cold
worked CRA's is not greater than 130ksi Duplex tubulars should show very good
service lives.
Similarly, early failures of 25Cr Duplex SS bolts was attributed to ferrite contents as high as 70% and due to heat treatment (475°C for 6 hours) to increase strength through formation of the α' - phase. This practice is no longer used, ferrite contents are reduced to about 50% and cold working is utilised to produce the required strength. There is no longer a problem of HE of modern Superduplex SS fasteners.
2. 'Effects of alloy composition and conditions of use on the performance of
copper-nickel sheeting for small boats', Mr
Hector Campbell, Consultant
Unexpectedly
severe corrosion of adhesive-backed 0.005" 90/10 Cu/Ni sheathing on two
yachts berthed in Chichester Yacht Harbour was shown to be due to the use of
material containing only 0.75% Fe instead of the specified 1.0-1.8% and the fact
that these yachts spent most of the time at their moorings. A third yacht,
sheathed with the same material and berthed in the same Yacht Harbour but sailed
frequently, was not affected. Exposure tests in Chichester and Emsworth Yacht
Harbours showed the low-iron material behaved satisfactorily under test
conditions involving moderate flow rates but not under very low flow conditions.
Material containing 1.5% iron was satisfactory under both conditions of test.
3. 'Ceramic/metallic corrosion resistant
coatings', Mark
F Mosser - Sermatech International
Coating
systems were described that combined the thermal stability of ceramics with the
corrosion resistance of active metals. Curing mechanisms were described, SEM
photomicrographs of structures were presented. Corrosion protective mechanisms
were described and coating properties listed. Applications of these products to
saline environments were shown and explained.
4.
'Application
of cathodic protection at limited potentials', Dr
Robin Jacobs - Global Corrosion Consultants
Invariably
cathodic protection is applied at -l000mV to -1100mV (Ag/AgCl/Seawater) due
to the choice of sacrificial anode material, and the need to protect carbon
steel.
However
at these typical potentials HE of high tensile steel and in the past DSS has occurred.
A novel method for the control of potential to levels required for SS was
described using Schottky barrier rectifiers which limit current flow to a 300mV
drop over the unit. Similarly, on applications where Mg sacrificial anodes were
required for current density, these rectifiers reduced the potential from -1500m
V to the required -1100mV for steel.
1.
‘Fundamental Characteristics of Pitting Corrosion’, G.L. Burstein
(Department of Materials Science and Metallurgy, University of Cambridge)
Pitting
corrosion is a problem characteristic of many metals in contact with
specifically aggressive anions, usually (but not always) chloride, in solution.
It is widely classified by measuring the pitting potential using a
potential scan. Measurement of current/potential characteristics of stainless
steel using a microelectrode shows that the passive slate is already unstable at
potentials well below the pitting potential.
This instability is manifest as a series a current transients arising
from the nucleation and propagation of metastable pits.
A metastable pit is one which propagates briefly and then dies through
repassivation. All pits, whether
they repassivate or become stable, commence propagation as metastable pits.
The metastable pit grows because of accumulation of metal cations and
associated chloride anions into its occluded region. The
rate of the process is under diffusion control, and the diffusion is
characterised by the product of the hemispherical pit radius, and its current
density, the pit stability product. The
transition from metastable to stable pit growth on stainless steel in chloride
solution occurs when the pit stability product reaches a critical value of 3 mA/cm.
Survival of the pit in the metastable state thus depends on the survival of a
porous cover over its mouth, a remnant of the passive film that originally
covered the site. Analysis of the
frequency of metastable pitting events shows that they arise from discrete sites
on the metal surface and are progressively annihilated.
By
using diluted chloride solutions to render the electrochemical noise more
"quiet", even metastable pitting becomes more difficult, and the
nucleation events can then be observed as sharp current spikes, clearly showing
a microscopically violent origin. These
events are the original depassivation processes that nucleate pitting. They
are tiny, observed largely in the
picoampere regime, and there are many of them. Most
pit nucleation events on stainless steel do not propagate, even into the
metastable regime, but die immediately after nucleation.
2.
‘Paint Systems for the Protection of Marine and Offshore Structures’,
Dr N R Whitehouse (Paint Research Association)
Surface
coatinga are used extensively to protect ships and offshore structures from
corrosion. This paper will review
the protective coating systems used on ships and offshore structures and
indicate some of the reasons why particular generic coating types are specified.
The
requirements of protective coatings for ships include:
-
Good anticorrosive properties
-
Good water resistance
-
Good durability
-
Good resistance to mechanical damage
In
addition, antifouling properties may be required.
Typical
paint systems for Underwater Parts, Boottop Zone, Topsides, Decks, and Tanks
will be described and rationalised in terms of performance requirements.
Fixed
offshore structures, because of their fixed nature, differ from ships and
present one of the greatest technical challenges to the surface coatings
Industry. Protection from corrosion
for more than twenty five years may be specified by operators, as maintenance
painting onshore is not practical and offshore is not only expensive but often
difficult to do well.
Typical paint systems for Jackets, Decks, Modules, High Temperature Areas, and Risers will be described.
3. 'Life Cycle Costing', D.J. Cochrane (NiDI)
The economic assessment of the performance of materials for a given project can be quantified by the use of the Life Cycle Costing (LCC) technique. A software tool developed by NiDI in conjunction with Euro Inox will be outlined in this presentation and illustrated with worked examples evaluating the cost of carbon steel with stainless steel for two different offshore applications.
1. 'Pros and cons of direct and indirect seawater cooling', Liane Smith, Intetech Ltd (NiDI)
Dr Smith discussed seawater cooling in the oil and gas offshore industry principally associated with gas compression trains. Direct cooling involves only one material interface between the seawater and gas, and has inherent advantages of maximum temperature differential between product and coolant and minimizes the number of components. However, there are disadvantages of handling a hot corrosive medium (requiring "exotic" materials) and fouling.
With indirect cooling there are two material interfaces; seawater/treated water and treated water/gas. Materials selection is often determined by gas corrosivity, but does not therefore need such highly alloyed materials. Disadvantages include less efficient cooling, greater space and weight, additional facilities to treat water.
The design considerations were then discussed in terms of impact upon factors such as fouling, scaling and pressure handling. In fabrication, front welding of tubes into tube plates is straight forward, but results in a crevice being produced. To overcome this, back-welding is possible, but at a higher cost.
Although the optimum design for low pressure or a single exchanger would appear to be a direct cooled, untreated seawater in tube with gas in shell design, it was not known whether such systems were in use. The majority of high pressure/temperature systems seemed to be indirect with treated water in shell, front face welded tubes and materials selection based on the gas.
In the intermediate regime, chlorinated seawater (0.2ppm Cl2) direct cooling in shell resulted in the highest acceptable metal temperature being governed by the material, e.g. 22Cr duplex is not used in the direct system due to insufficient pitting resistance. Materials such as 254SMO (6Mo SS) could be used but only up to 40°C. However, a new system is soon to enter service using C276 allowing an 80°C maximum temperature, with a shell of 625 weld overlay on carbon steel.
Future developments currently being researched are the use of "compact coolers" of stacked grooved (lmmØ) plates (2-3mm thick) leading to exchanger volumes of 1/m3 equating to cooling capacities of tube bundles of 6m length.
2. 'Biodegradation of materials', Ken Tiller, Nat Corr Service (DTI)
Mr Tiller described the nature of organisms which can lead to microbiologically induced or influenced corrosion of materials. Three broad categories of organisms were defined as: mesophilic, halophilic and thermophilic. As an environment changes so the population balance will alter as conditions favour one species over another. SRB cannot be killed, but become dormant until conditions again become favourable. Bacteria can actually modify themselves to suit the environment, by, for example, developing hydrogenaze to facilitate the use of hydrogen (e.g. where cathodic protection or corrosion generates adsorbed atomic hydrogen H°). They can also become tolerant of biocides, wide temperature ranges and pressure changes.
Organisms can either induce corrosion due to their by-products, e.g. exopolymeric films, or locate at areas of corrosion due to the favourable conditions so produced. All materials appear to be affected in a wide range of environments, although titanium may be an exception. Systems can often be contaminated due to the use of dirty water for hydrostatic testing.
MIC can sometimes be observed by the corrosion morphology, e.g. increased Fe/Mn at a corroded weld in steel, or pits on DSS appearing canyon-like. Bacteria are split into those in the sulphur, nitrogen or carbon analyses. SRB lead to cathodic depolarisation due to the removal of H° and iron sulphide film formation. The type of sulphide formed will also significantly affect corrosion rates on ferrous materials, pyrite and greigite accelerating attack.
Various techniques are used to identify the presence of MIC, e.g. API RP38, electrochemical methods, bioprobes. Prevention or control may be achieved by a range of measures; material upgrade, use of non-metallics, coatings, CP, biocides and biostats, aldehydes, alkaline conditions. In summary, this is an area of corrosion which is still being investigated, but which is obviously of great importance.