Technical Presentations at the July 2011 Meeting

1.1   Mechanisms of Localised Corrosion of Aluminium Alloys’, Alison Davenport, University of Birmingham 

It is well known that intergranular corrosion of Al-Mg alloys is caused by dissolution of Mg-rich β-phase particles precipitated on grain boundaries.  However, TEM studies and SEM examination of Ga-induced fracture surfaces have shown that these are generally discontinuous.  Electrochemical evidence suggests that dissolution of  β-phase particles releases aggressive Mg2+ ions into the cavity, promoting local corrosion of the aluminium matrix in between β-phase particles, leading to continuous intergranular attack despite the discontinuous nature of the particles.   The aggressive nature of solutions rich in Mg2+ ions may also explain the role of Mg2Si particles in pit initiation in Al-Mg alloys.  Friction stir welding of Al-Mg alloys causes fragmentation and dissolution of these particles, leading to improved pitting resistance.  Friction stir welding and friction surface processing can also decrease the risk of intergranular corrosion and could potentially be used as a repair technology for sensitised structures.   

Atmospheric pitting corrosion of Al alloys under NaCl deposits is enhanced by “secondary spreading” of the alkaline edges of salt droplets: this is inhibited by Mg in the alloy or in the salt, indicating that electrochemical measurements under full immersion conditions may not always offer an appropriate testing method for atmospheric corrosion conditions.  Instead, a new approach using inkjet printing to deposit salt patterns on metal surfaces to form droplets as the humidity is raised may be a more controlled method for testing for atmospheric attack.  

Synchrotron X-ray microtomography is a very promising technique for observing localised corrosion of aluminium alloys, particularly under atmospheric corrosion conditions, since both the size and shape of salt droplets and the penetration of corrosion into the interior of the alloy can be measured in situ and in real time. 

1.2   Corrosion vs. Safety vs. Sustainability - The Responsibility of the Corrosion Engineer (Examples in some aluminium alloys)’, Chris Wheatley, CJ Wiretech Ltd 

The role of the Corrosion Engineer was examined in a case study involving the use of a thermally sprayed aluminium-titanium alloy to form a non-slip coating for steel in a marine environment.  The coating was tested for frictional performance with pedestrians and was found to be ‘highly anti-slip’ in line with the British and European norm.  Then wear by a robotic walking machine demonstrated that the anti-slip state continued for over 1,000,000 footfalls.  To a similar end, the Taber Wear Resistance Test demonstrated that the coating had half of the wear of sprayed aluminium.  

The coating was also tested for its resistance to corrosion in a saltspray chamber for 1000hr and the results showed that the aluminium-titanium alloy resisted self-corrosion much better than pure aluminium without a corresponding loss of performance in the galvanic protection of the exposed steel scratch.  This was backed up by measurements on the coating using a cell incorporating a zero resistance ammeter, where the alloy was shown to be about 100mV nobler than pure aluminium whilst giving an identical galvanic current in the protection of the steel.

It was argued that paints could also be used to provide a non-slip surface but that painting was extremely hit and miss and that paints would not give the same galvanic protection to exposed steel. The paint industry had still to solve the problems relating to the use of heavy metals in primers.

The case study demonstrated that the Corrosion Engineer needed to give to his client a balanced view on lifetimes of prospective solutions to corrosion problems – the client might ask for a cheap option but the Corrosion Engineer had a responsibility to point out other, more expensive solutions and to remind the client of the need for Sustainability of processes in the future.  As an aid to this procedure the Corrosion Engineer could invoke the norm ASTM A1068-10 to provide a template for offering the information.

[A pdf version of this presentation is available to members on request to the Secretariat].

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3.1  Corrosion Protection of Offshore Structures Using Coatings’, Shiladitya Paul, TWI 

Offshore operators (both oil and gas and wind energy) are currently looking to extend the design life of offshore facilities, structures and components to improve the affordability, and to increase their availability in later years of operation. Whilst maintenance and replacement of topside facilities is possible, critical to this objective is the design and construction of supporting infrastructure and facilities capable of withstanding splash and tidal zone conditions, particularly corrosion, for the lifetime of the structure with the absolute minimum of maintenance. 

To reach the above objective, TWI launched a Joint Industry Project (JIP) in 2009 wherein a technology gap review was carried out with the help of industrial and academic partners to establish current working practice and experience related to the mitigation of splash and tidal zone corrosion. A number of coating systems (thermally sprayed, organic paints and duplex) currently used in the splash and tidal zone of offshore structures were identified. Samples based on the chosen coating system were prepared and tested in both seawater spray (modified ISO 9227) and alternate immersion tests. LPR and EIS measurements were carried out during the tests and after the test the samples were cross-sectioned and microstructural characterisation was undertaken. 

The observations were briefly described in the presentation. 

[A pdf version of this presentation is available to members on request to the Secretariat].

3.2    Hybrid Electrochemical Treatment of Marine Concrete Structures’, Gareth Glass, Concrete Preservation Technologies

 

This presentation addressed the challenges of in-situ concrete repair and corrosion monitoring.   Corrosion damage is at least in part attributed to the production of acid (HCl) at sites of corrosion initiation.  Pit re-alkalisation is identified as an important protective effect in electrochemical treatments used to arrest corrosion.  The process of pit re-alkalisation may be achieved using a relatively small electric charge that is readily impressed off sacrificial anodes using a power supply.  A simple but powerful electrochemical treatment comprises a hybrid of a brief pit re-alkalisation process to arrest corrosion followed by low maintenance galvanic protection to prevent local acidification.  Both reinforced and pre-stressed concrete were discussed and examples were provided in a variety of structural applications using proprietary electrochemical methods to extend service lives and reduce maintenance costs.

 

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