Technical Presentations at the July 2005 Meeting

2.1  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 research literature.  

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 energy. 

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.

2.2      ‘Welding Issues with Copper Alloys’, Mike Gittos & Chris Punshon (TWI) 

This presentation covered the aspects of welding copper and its alloys including the most commonly used welding techniques and the various issues with welding the different metals and alloys. Corrosion issues were highlighted here appropriate. The second half of the presentation focussed on canister fabrication for nuclear waste storage in Sweden, involving reduced pressure electron beam welding and friction stir welding.    

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4.1   Long-term and Accelerated Corrosion Performance of Cast Nickel Aluminium Bronze in Seawater’, Robin Oakley and John Galsworthy (QinetiQ) 

Part 1 :  Long-term and Accelerated Corrosion Testing Methods for Copper-Based Alloys in Seawater’, R.S. Oakleya*, J.C. Galsworthya, K.R. Stokesb, G.S. Foxa  

[aQinetiQ Ltd, Cody Technology Park, Farnborough, Hampshire, UK. GU14 0LX; bDefence Science Technology Laboratory, Porton, Salisbury, Wiltshire, UK. SP4 0JQ] 

There is 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 methods.  [ÓQinetiQ Ltd 2005] 

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]  

4.2    Learning from Failures’, Derek Bates (DRB Materials Technology Ltd) 

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 example. 

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 Rig. 

The Mechanism of Failures include:

  • Welding as a cause of failure
  • Poor understanding of materials characteristics
  • Fatigue as an initiator of failure
  • Corrosion, Structural overload.

The primary 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: 

Ignorance, Apathy, Management indecision, Negligence, Over confidence, Poor communication, Error & misjudgement, Pre–occupation, Level of Uncertainty, Inaccuracy of data, Poor education & training, Vandalism & terrorism.

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