Technical Presentations at the January 2012 Meeting

1.1  ‘Synchrotron Light - Novel Techniques for Studying Corrosion Mechanisms’, Trevor Rayment, Diamond, University of Birmingham   

The Diamond Synchrotron X-ray facility in Oxfordshire was introduced as a source of high-brightness sub-micron X-ray beams which is generally available for use in the study of complex inhomogenous materials and systems under realistic conditions.  The combination of the brilliance of the highly developed synchrotron source and optics are able to focus the beam to a micron sized spot, allowing compositional, temporal and spatial information to be gathered at high resolution. 

Using this very controlled X-ray source researchers have been able to map elements in complex samples, follow chemical reactions, study real systems such as mineral samples returned from space and analyse environmental samples and materials in hostile environments.  Trevor Rayment, together with Alison Davenport and her team at the University of Birmingham have used the technique to carry out X-ray studies to examine the phenomenon of pit corrosion. 

Corrosion pits in stainless steel penetrate beneath the surface of the otherwise passive metal. Previous studies have examined the formation of salt films in the pits but the chemistry and structure of the salt layer has been difficult, as the layers are present only on dynamically dissolving metal surfaces. 

The work presented used artificial pits, created by embedding an alloy wire in epoxy resin and dissolving it back.  This created a one-dimensional corrosion pit containing a highly concentrated acidic metal chloride solution characteristic of real pits but which could be directly observed with a micro-focused synchrotron x-ray beam.  The study was the first to determine directly the structure of salt films formed on dissolving iron, nickel and stainless steel surfaces.   The results showed that the micro-structure of the salt films formed on iron and stainless steel are quite different.  The smooth, continuous diffraction rings observed on iron indicate many fine, randomly oriented crystallites. In comparison, on 316L stainless steel the rings contain a few very intense diffraction spots, indicating fewer, larger crystallites.  The crystallite size has an impact on the transport properties of the salt film, and the roughness characteristics of the metal surface.   CT

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1.2 ‘Corrosion Testing Protocols for Ship Ballast Tanks’, Tim Illson, G L Noble Denton

Recent changes to international marine regulations require water within ship ballast tanks to be treated to prevent the transport of invasive organisms.  Potentially the active treatments such as ozone or chlorination could influence the corrosion behaviour of tank materials.  Therefore, all new treatment processes must show by corrosion testing that they do not threaten the integrity of the ballast tanks.  

The presentation described how such a corrosion test programme was planned and executed.  It explained the difficulties encountered in performing and monitoring the tests and presented an outline of the results.  It was found that the system tested did not increase integrity risks for conventional ballast tank materials and coatings.   [Contact: tim.illson@gl-group.com]

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3.1    Application of Surface Treatments for the Enhancement of Corrosion Resistance’, John Yarnall, Bodycote Hardiff BV & Phil Dent, Exova

Part 1: S3P Kolsterising®, John Yarnall

Many tribological and surfaced stressed applications demand that the components be highly resistive to wear and corrosion.  

Austenitic stainless steels meet the corrosion resistance required in most environments.  However, the use of austenitic stainless steels is limited due to the low hardness and  low wear resistance with the risk of galling.  This family steels cannot be surface  hardened  by standard heat treatment processes without reducing their corrosion resistance.  As a consequence there is limited scope to extend their application  range.  In such cases, Bodycote's unique Kolsterising surface hardening  treatment  can  be applied to meet  most application  requirements to mitigate surface wear, and  provide enhanced  resistance in most corrosion environments. 

Kolsterising® does not apply a coating to surfaces, but produces a pure carbon rich diffusion zone from  the surface inwards, with excellent toughness and no risk of delamination or peeling. The treatment  increases the surface hardness of most austenitic stainless and CRA steels to a level of about 1000 to 1200 HV0.05 (depending on base material and surface conditions). Moreover, Kolsterised® components exhibit excellent fatigue properties due to high compressive stresses produced during the surface hardening process.  The treatment is a low-temperature process causing minimal changes to geometry and shape.  

Kolsterising® is the chosen treatment for many applications in most industrial areas including food, marine, engineering, medical, chemical, nuclear and oil/gas extraction industries.  Wherever components are subjected to severe wear and corrosion, Kolsterising® is one of  the most technically advanced hardening process offering the required properties to meet the cost effective needs demanded by industry.  [Contact: john.yarnall@bodycote.com] 

Part 2: Corrosion Testing of Kolsterised Austenitic & Duplex Stainless Steels, Phil Dent

Phil Dent presented the results of testing undertaken by Exova on kolsterised 316L austenitic stainless steel and 22%Cr and 25%Cr duplex stainless steels.  The pitting resistance was evaluated using the ASTM G48 method and the resistance to stress corrosion cracking (SCC) in sour environments (H2S) was assessed against the standard conditions in ISO 15156 / NACE MR0175.  The influence of the kolsterising surface treatment on the microstructure and mechanical properties was also determined. 

The results of the tests were presented which showed that the kolsterising surface treatment increased the critical pitting temperature (CPT) of the 316L stainless steel and the 22%Cr duplex stainless steel, with no adverse influence on the microstructure and impact properties.  The kolsterising treatment did not influence the SCC resistance of the 316L stainless steel and was found to be beneficial to the SCC resistance of the 22%Cr stainless steel.  The kolsterising treatment did have an adverse influence on the CPT and charpy impact resistance of the 25%Cr duplex stainless steel, although no evidence of intermetallic phase precipitation was present.  The reduction in the impact properties of the 25%Cr duplex stainless steel was considered to be attributable to the formation of alpha-prime during the kolsterising treatment.   

Further testing is planned to characterise the influence of the kolsterising treatment on the hydrogen induced stress cracking (HISC), sulphide stress cracking (SSC) and seawater resistance of 316L stainless steels and 22%Cr duplex stainless steel.  [Contact: phil.dent@exova.com]

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3.2   Practical Aspects of Stainless Steel Construction for Marine Applications’, Clive Tuck, Lloyd’s Register EMEA

Stainless steels rely for their corrosion resistance on a thin protective chromium-rich oxide layer of about 1nm thick.  This is difficult to maintain in a flaw-free state and, in an environment containing chloride ions, it suffers continuing attack in which the passive layer can be destroyed.  Research work in the 1980’s identified that the geometry of the flaws in the passive layer determined whether, after this destruction, the passive film would re-form or whether localised corrosion would develop which would result in the formation of pits on the surface.  The work recognised that the pitting process could be described through the use of statistics and, because attack by chloride ions was a continual process, it was estimation of the risk that the attack would develop into pits which was the important factor in the application of stainless steels to engineered structures.  This presentation deals with using construction methods for stainless steels which minimise the risk of their corroding and the key proposition put forward is that the workshop practices used must ensure that the passive film on the surface is kept free from damage. 

Primarily, stainless steel surfaces should not be allowed to become mechanically impaired and the handling practices and working environment need to be cleaner and more ordered than those used for carbon steel.  Stainless steel should not be allowed to come into contact with carbon steel and grinding dust or weld spatter from carbon steel operations should not be allowed to settle on the surface of stainless steel.  Marker pens for identification of stainless steel should be specified to be chlorine and chloride free.

Welding practices need to be carefully managed, as the different grades of stainless steel have markedly different welding procedures.  Excessive surface oxidation must be avoided during the welding process through the use of inert shielding and backing gases.  When welding stainless steel to carbon steel, the filler material compositions need to be chosen carefully in order to produce welds with optimised mechanical properties and corrosion resistance.

After welding, post weld heat treatment (for stress relief) is really only applicable for martensitic stainless steels and the surfaces of all the grades of stainless steels need to be carefully cleaned.  For general surface cleaning, grit blasting or garnet blasting should be used.  In order to encourage a well-formed passive oxide layer, it is recommended that a chemical passivation treatment is given. 

 

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