Philippe Marcus outlines a new vision of corrosion science, exemplified by the ERC CIMNAS project: Corrosion Initiation Mechanisms at the Nanometre/Atomic Scales’.
Ageing, degradation, and failure due to corrosion is a major problem for our advanced society as it affects numerous technical fields and industrial sectors in which metal and alloy components are used for their structural or functional properties. This includes energy production and transport, the oil, gas and chemical industries, infrastructures, automotive, aeronautic and naval transportation, biomedical implants and even microelectronics, to name but a few. Not only are the performance and durability of components or infrastructures reduced by corrosion with a very high economical loss (a cost of about 4% of the yearly gross national product is estimated), but safety can be compromised in applications where it is mandatory, such as in aeroplanes or nuclear power plants, or where the environment can be put at risk in, for instance, oil and gas transport. Even health can be at risk with regard, for example, to bioimplants, which would leach metal cations.
Corrosion takes various forms depending on the nature of the material and environment.
Despite this complexity, corrosion science is well advanced, thanks to an ever-growing number of studies performed to investigate corrosion phenomena and to test and validate the various means to mitigate corrosion and to develop corrosion protection.
The mechanisms of corrosion propagation are fairly well understood, although research is still needed in this area, and while various means of mitigation are now being utilised, but progress is still needed.
A new vision for corrosion science
Corrosion phenomena initiate on the material’s surface, and the initiation occurs at the atomic or nanometric scale. The new vision is based on the idea that blocking or retarding the initiation at the relevant scale (atomic/nanometric) will in the future be the way to solve or mitigate corrosion problems. To this end corrosion initiation mechanisms must be well understood at the relevant scale.
The nature of weak sites at the surface where corrosion initiation takes place must be uncovered and the details of the initiation mechanisms must be elucidated. This is the challenge addressed by the ERC Advanced Grant project CIMNAS. Developing such a deep understanding will then open the way to the design of new materials, the invention of new processes, and the development of surface nanoengineering targeting the sealing or the inhibition of the local initiation sites on material surfaces.
In order to exemplify this approach, one of the three axes of the ERC CIMNAS project is briefly presented here: it bears on structural and chemical heterogeneities/defects produced by the oxidation process itself. Indeed although a surface oxide film (called ‘passive film’) is necessary to protect metals and alloys against corrosion, the oxidation process preceding passivation may cause by itself the appearance of structural/chemical heterogeneities/defects that may later trigger the initiation of localised corrosion. This is investigated in details in the ERC CIMNAS project.
The other axes of the CIMNAS project cover:
- The competition between localised dissolution and passivation of surface terminations of grain boundaries; and
- Research on key factors that make an organic molecule a good corrosion inhibitor on surfaces that may be incompletely passivated, meaning that both oxidised and reduced metallic areas co-exist.
Resources at CNRS-Chimie ParisTech
Understanding the corrosion initiation phenomena at the nano- or atomic scale requires a research effort that not only uses state-of-the-art analytical techniques providing relevant surface and interface information at these scales, but also necessitates designing of model systems that are relevant for the complexity of the processes and interfaces at play. Experimental research should be performed at the relevant scale, which may be the nano- or atomic scale, depending on the issue being addressed, and should be systematically complemented by atomistic modelling.
Such research is conducted by the team at the Physical Chemistry of Surfaces of CNRS – Chimie ParisTech (Institut de Recherche de Chimie Paris). This team has expertise in the approach of corrosion phenomena based on detailed chemical and structural characterisation of surfaces and interfaces at the nanoscale combined with electrochemical measurements, and in establishing the link between mechanisms at the nanoscale and macroscopic manifestations of corrosion phenomena.
Technical resources include a new apparatus for in situ surface analysis, imaging and electrochemistry under gas and liquid environments (see Fig. 1). It combines, in the same system, in situ XPS (X-ray Photoelectron Spectroscopy), in situ STM (Scanning Tunnelling Microscopy), and electrochemistry with direct transfer of samples in the closed system without exposure to air. This facility allows for the application of surface analysis techniques far beyond the usual level. XPS and STM can be combined for in situ measurements in gas, and STM for in situ measurements in liquid with simultaneous electrochemical measurements of the reaction kinetics, all integrated into one system. Cross analysis with sample transfer under inert gas in the closed system and to other closed systems without exposing the sample to air can also be performed.
Other technical resources include a stand-alone platform for STM/AFM analysis in liquid environments under electrochemical control and ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) equipped with a direct transfer to an Ar-filled glove box.
A key issue: corrosion initiation by local passivity breakdown
Passivable alloys, and among them stainless steels, are widely used in our industrial society because they self-protect against corrosion by forming nanometre thick barrier oxide films enriched in chromium at the surface. However, under some environmental conditions this ultra-thin protection, the passive film, can fail by local breakdown, which can lead to localised corrosion. One of the key issues for corrosion initiation is the role of local disruptions of chemical and structural homogeneity that may be produced by the oxidation mechanisms in the early stage of oxidation of alloys.
The new vision here is that the pre-passivation mechanism (formation of the native oxide) may produce the chemical heterogeneities leading to the initiation of localised corrosion. Indeed, as inferred from recent observations on stainless steel, the nucleation and growth mechanisms in the pre-passivation stage would produce different levels of chromium enrichment varying between the oxide nanograins themselves and also depending on the co-ordination sites (atomic steps vs. terraces) of the substrate, thus creating local chemical conditions favourable to trigger localised corrosion.1, 2
The factors governing passive film stability on stainless steel, so far unknown at the nanometric/atomic scale, can be interrogated using this surface science approach of corrosion initiation.3 Starting from an oxide-free model surface of austenitic stainless steel (see Fig. 2), nanoscale heterogeneities, with variations of composition and structure, appear in the surface oxide film.4
The nanoscale heterogeneity of the surface oxide originates from the mechanism of chromium pumping from atomic terraces to the multi-steps for preferential formation of Cr(III) oxide nuclei. Terrace borders adjacent to step edges may thus be chromium depleted and iron-rich islands can be formed. The barrier properties of the passive film can be affected by this heterogeneity. Such detailed work allows us to better understand the factors governing the stability of protective surface oxide films. It brings important new insight into the nature and the mechanisms of formation of surface oxides and their impact on the initiation of localised corrosion, and provides the basis for new opportunities to design treatments to preclude passivity breakdown by acting on pre-passivation.
The effects of grain boundary terminations at material surfaces on local dissolution and passivation properties is another key issue for corrosion initiation. Their reactivity has been generally associated to the segregation of impurities (e.g. sulphur or phosphorous) or to the presence of precipitates (carbides, sulphides). Now we must make progress towards a better understanding of the intrinsic reactivity of grain boundaries.
The CIMNAS project addresses the mechanisms of dissolution and passivation of grain boundary terminations, for simple boundaries (twin boundaries) as well as for a variety of more complex boundaries. A novel methodology combining ex situ microstructural and in situ nanostructural and electrochemical characterisation is being developed. The success in elucidating the mechanisms of dissolution/passivation of grain boundaries could be the basis for developing ways of controlling the orientation of grain boundaries at the surface, and/or curing the surface defects associated with the grain boundary termination by a passivation process of the grain boundary emergence sites.
Understanding the mechanisms of inhibition of corrosion initiation of surfaces not uniformly passivated is also a new challenge addressed by the CIMNAS project. It aims at understanding the adsorption of organic inhibitors on surfaces exposing both oxide and metallic areas in order to elucidate the long lasting issues of:
- The metal chemical state involved in the activity of the inhibitor; and
- The key factors for inhibiting corrosion of both metallic and oxide areas co-existing on the surface
The expected impact is to find the knowledge-based guidelines for predicting effective inhibitors working on partially oxidised surfaces (i.e. inhibiting the metal, the oxide and the periphery of the oxide patches, thought to be critical for corrosion initiation).
With this new vision of corrosion science, the progress in corrosion chemistry at the nanometre or atomic scale, which is the scale at which corrosion initiation takes place at material surfaces, will provide the necessary basis for the development of new processes and new materials with improved durability and extended life time. The benefits will be cost reduction, increased reliability of industrial installations, and the improved safety of people and health.
1 V. Maurice, P. Marcus, ‘Progress in corrosion science at atomic and nanometric scales’, Progress in Materials Science 95, 132 – 171 (2018)
2 V. Maurice, P. Marcus, ‘Current developments of nanoscale insight into corrosion protection by passive oxide films’, Current Opinion in Solid State and Materials Science 22, 156 – 167 (2018)
3 L. Ma, F. Wiame, V. Maurice, P. Marcus, ‘New insight on early oxidation stages of austenitic stainless steel from in situ XPS analysis on single-crystalline Fe-18Cr-13Ni(100)’, Corrosion Science 140, 205–216 (2018)
4 L. Ma, F. Wiame, V. Maurice, P. Marcus, ‘Origin of nanoscale heterogeneity in the surface oxide film protecting stainless steel against corrosion,’ to be published.