Technical Presentations at the January 2012 Meeting
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.
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.
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
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
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: [email protected]]
1: S3P Kolsterising®, John Yarnall
Many tribological and
surfaced stressed applications demand that the components be highly
resistive to wear and corrosion.
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
be applied to meet most
application requirements to
mitigate surface wear, and provide
enhanced resistance in most
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
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: [email protected]]
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
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: [email protected]]
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.