Technical Presentations at the July 2005 Meeting
‘Corrosion Performance of Nickel
Aluminium Bronze (NAB)
Under Seawater Immersion and Erosion-corrosion Conditions’, Rakesh Barik, Julian Wharton & Robert Wood (Surface Engineering
& Tribology Group, School of Engineering Sciences, Southampton
University) & Keith Stokes (DSTL)
Cast nickel-aluminium bronze
(NAB), with a nominal composition of CuAl10Fe5Ni5, is extensively used in
seawater environments because of its
castability, high toughness and erosion-corrosion resistance.
However, service experience has shown that it may encounter
corrosion related problems. It is known that NAB
has an inherent susceptibility to selective phase corrosion due to the
complex microstructure formed on cooling.
In addition, NAB can encounter variability in corrosion performance
worldwide, i.e. different local environments.
NAB may be affected by the seawater environmental factors,
which include biofouling, flow rates, temperature, salinity, pH and
dissolved oxygen contents. Thus,
the current work examines the effect of seawater on the corrosion
performance of NAB when galvanically coupled to either NAB or Ti, an area
where little data exists, especially with regard to the influence of
surface oxide films/deposits and biofouling.
Accelerated corrosion rates were measured for NAB/Ti couple compared to
NAB/NAB couple particularly during the biofouling season.
This behaviour coincided with the seasonal increase in
seawater temperature and increased likelihood of biofilm formation and
activity. The role of
bacterial metabolites/enzymes in the biofilm have been reported to modify
reduction kinetics and could be responsible for the increase in corrosion
activity. Although the corrosion
rates for NAB/NAB couple was similar to reported values found in the
To investigate the influence
of flow environments, single and two-phase (sand particle entrainment)
have been made to assess the mechanical damage of NAB components.
The erosion and erosion-corrosion behaviour of NAB is investigated
under a range of kinetic energies from 0.02 to 0.40 mJ
and compared with thermally sprayed NAB which a possible candidate used
for refurbishment of marine components.
The synergistic effect based on the interaction of erosion,
corrosion and erosion-corrosion experiments are also presented.
NAB has good erosion-corrosion resistance at low kinetic energies
but higher kinetic energy results in accelerated attack.
However, the thermally sprayed NAB
coating was inferior to the cast NAB under these conditions.
Comparison of the erosion performance with that of the
erosion-corrosion performance both sprayed and cast NAB surfaces
demonstrate a propensity for negative synergy.
However, vulnerability at high energies to positive synergy was
seen for the coating. The
option of using thermally sprayed NAB is a viable one for low kinetic
energy applications but further improvements in the coating are required
before this option can be applied to high
The nature and presence of a
protective oxide film is fundamental to the overall corrosion and
erosion-corrosion performance of NAB.
Under seawater immersion
conditions the protective oxide film will form and mature with time
resulting in improved corrosion resistance.
For erosion-corrosion, the ability to form an adherent protective
oxide under low kinetic energies may result in a reduction in the wear
rate, especially when the oxide layer remains intact under particle
impingement. At higher
kinetic energies NAB is more vulnerable to erosion-corrosion due to
degradation processes, where continual stripping and/or rupturing of the
oxide film can occur.
Part 1 :
‘Long-term and Accelerated Corrosion Testing Methods for
Copper-Based Alloys in Seawater’,
R.S. Oakleya*, J.C. Galsworthya, K.R. Stokesb,
Ltd, Cody Technology Park, Farnborough, Hampshire, UK. GU14 0LX; bDefence
Science Technology Laboratory, Porton, Salisbury, Wiltshire, UK. SP4 0JQ]
extensive literature available on the evaluation of the localised corrosion
resistance of stainless steels and nickel alloys. However, less attention has been applied to the understanding
of pitting and crevice corrosion in the copper-based alloys used for many
seawater service applications. Long
periods of seawater exposure are often required for reliable evaluation of the
localised corrosion performance of naval copper alloys such as aluminium bronzes
because some degradation mechanisms initiate only after significant periods of
immersion. The impact of different
temperature and seawater chemistry on copper alloy corrosion can be difficult to
simulate when natural exposure is used as a primary test method.
The results of
natural seawater exposure tests on nickel aluminium bronze will be reported.
Rapid electrochemical methods originally established for testing
stainless steels, have also been applied to this material.
A comparison of the results will be made to determine the effectiveness
of such accelerated testing for copper alloys.
The degradation mechanisms of the copper alloys will be reviewed and
trends in their performance identified, with possible contributory factors
discussed and proposals made for future developments in corrosion testing
Part 2: ‘Some
observations made during the destructive examination of cast nickel aluminium
bronze seawater system components after service’, John C. Galsworthy and Robin S. Oakley, QinetiQ Ltd., Cody
Technology Park, Farnborough, GU14 0LX, UK
Cast nickel aluminium bronze (NAB) components are used extensively within seawater systems. NAB was originally introduced to replace gunmetal which was considered to have poor toughness and performance under shock loading conditions. However, although NAB has good toughness and reasonable general corrosion performance, it has been found to suffer from selective phase corrosion (SPC). This has necessitated the careful management of these castings in service, involving initial validation and subsequent revalidation of all castings to ensure continued suitability for service. The procedures behind the management of this alloy within the fleet requires a knowledge of its performance during service. This was originally collected through the destructive examination of ex-service components from earlier platforms. However, no data had been collected on new platform designs. A recent opportunity arose to destructively examine a number of castings removed from a submarine platform at its first long overhaul period. This paper reports some of the observations made during the examination of these castings. The corrosion performance of individual castings varied considerably. There was a wide range of maximum corrosion depths determined during the examination. Some corrosion attack was localised within the castings. Examples of these variations in attack will be given and the implications of the results discussed. [ÓQinetiQ Ltd 2005]
The presentation discussed a broad
spectrum of failures which have resulted in litigation claims involving loss of
profitability, breakdowns, disasters, company failures and fatalities.
The content is aimed at extending the background of into areas of
engineering and design which are not common knowledge.
This is intended as introductory in nature and can be followed at a later
date by in-depth lectures on particular technologies, welding, corrosion, for
The topics covered centred on
construction but were also be taken from a range of alternative industries which
amplify the erroneous decision making which resulted in the failures.
The incidents covered some of the
following: Construction, Structures, Civil Engineering, Refineries, Offshore Oil
The Mechanism of Failures include:
lessons learned are that events do not happen in isolation – structures do not
collapse –materials do not fail etc. without human intervention.
With the exception of Acts of God, all failures are the result of human
error, resulting from: