It has been known for many years that the corrosion resistance of stainless-steels is dependent on the alloying elements, and especially chromium content, which provides the self-healing chromium oxide layer developed on the surface of the material in the presence of oxygen.  Historically, this layer has been presented as a thin, continuous, even and impenetrable layer achieved through chemical cleaning, pickling, and passivating treatments applied to the stainless steel after manufacture.  These pickling and passivating treatments typically rely on the use of reducing and oxidising acids to remove embedded iron contaminants and then promote the formation of the chromium oxide layer.  New research is shedding light into the formation and nature of the oxide layer and giving greater insight into the layer. 

From microstructural and crystallographic perspectives, a crystalline metallic material (i.e. most metals, but for cesium, gallium and mercury) is characterised by randomly orientated polygonal grains in which the atoms are arranged in a nominally uniform grid/crystal lattice.  The three-dimensional orientation of the lattice varies from grain to grain.  Furthermore, the structure (i.e. the packing of the atoms in the lattice) varies depending on the material, alloy content, and cooling rates/heat treatment.  Depending on alloy content (C, Cu, Cr, Mn, Mo, N, Ni, Ti) and cooling rates, stainless-steel can have i) face centred cubic (FCC) austenitic, ii) body cantered cubic (BCC) ferritic, iii) body cantered tetragonal (BCT) martensitic, or iv) duplex (mixed ferritic and austenitic) structures.  The structure of the material affects both its mechanical and corrosion performance.  Austenitic stainless-steels have good general corrosion resistance, ferritic stainless steels have relatively good corrosion resistance, but have lower tensile properties, martensitic stainless-steels have high tensile properties but have lower corrosion resistance, while duplex stainless-steels have excellent corrosion resistance and relatively high strength.  In addition, the structure also affects the magnetic properties, with austenitic stainless steels being non-magnetic (the common test to see if a piece of material is indeed stainless steel will only identify austenitic (and to some extent duplex) stainless steels).

In the austenitic FCC structure, which typifies AISI 304 and AISI 316 alloys, iron atoms are located in a regular three-dimensional lattice with atoms on the corners and centres of the faces of the ‘cubes’ formed by the lattice.  Atoms of the larger ‘substitutional’ alloying elements (Cr, Mn, Mo and Ni) displace some of the iron atoms in the lattice, while limited amounts of smaller ‘interstitial’ alloying elements (C and N) can fit between these atoms.  In the case of body cantered cubic structures, such as AISI 409 and AISI 440, the iron atoms are located on the corners of the grid with a single atom in the centre of the cube.  These atoms can be replaced by atoms from the alloy elements in a similar manner for the FCC structure.  However, although the packing density of the FCC structure is higher than that of the BCC structure, the interstitial spaces in the FCC lattice are bigger, allowing a higher concentration of interstitial alloying elements between the iron atoms than in the case of the BCC structure.  The body cantered tetragonal structures, which are typical of AISI 420 and AISI 431, are similar to the BCC structures except the grid is elongated in one dimension as opposed to having a cubic structure.

Sectioning through a piece of material cuts through the lattice of the individual grains and, depending on the angle of the lattice to the cut plane, the surface formed by such a cut will be stepped, or terraced, on the nanoscale with the cut edges of the lattice forming steps/terraces.  Any such sectioning process will expose the atoms in the lattice on the plane of the cut to the environment.  In the case of the AISI 304 and AISI 316 alloys, nominally 70% of these atoms will be iron, 16-20% will be chrome, and 8-14% will be nickel, with a small percentage of other elements, depending on the alloy.  Although the exact concentration of alloy elements in other alloys will differ, the percentage of iron will typically be greater than 70% (except in the super-austenitic alloys, where iron concentration can drop to below 40%).  This percentage of alloying element will be affected to some extent by ‘segregation’, which will result in alloying elements diffusing to the surface.  This time dependent diffusion process will affect the concentration of alloying elements at the surface, especially at elevated temperatures, and results in a reduction in the concentration of alloy in the core material.  Depending on the nature of the environment, the atoms on the surface of the exposed lattice will react with the environment.

Recent ground-breaking studies [1] have shown that in oxygenated environments, the oxygen reacts within nanoseconds at atmospheric pressure, with both the chrome and the iron on the surface forming nodules of chrome and iron oxide on the exposed surface.  These studies (undertaken at 250°C) show that the chromium on the edges of the terraces reacts first, and chromium from the bulk material and surface of the terraces moves to the edges of the terraces.  This is followed by the competitive formation of nodules of iron oxide (FeIII) between the chromium nodules, and with the formation of iron and chromium oxide on the surfaces of the terraces.  With increased oxygen exposure (10nS), the oxide nodules grow laterally to form a more compact and better ordered hexagonal structure, driven by segregation of chromium from the substrate, and an increase in the height of the iron oxide structures.  Further growth of the oxide (100nS) is characterised by iron oxide formation, driven by cation transport/diffusion to the outer surface through the oxide layer.  As iron ions diffuse faster than the chromium ions, through both the iron and chromium oxide, the inner layer of oxide is chromium rich, while the outer layer is iron rich.

These and other studies show that the protective oxide layer on stainless steels i) forms spontaneously and rapidly on clean/uncontaminated surfaces when they are exposed to oxygen, ii) is not homogeneous, at the nanoscale, and iii) comprises two layers – namely an inner chromium-rich layer and outer iron layer a few nanometers thick.

Although only a few nanometres thick, this self-healing layer of chrome and iron oxides gives stainless steel its corrosion resisting properties and will form spontaneously on exposure to oxygen if the surface is clean and not contaminated.

References:
1. Li Ma1, Frédéric Wiame 1, Vincent Maurice 1 and Philippe Marcus, ‘Origin of nanoscale heterogeneity in the surface oxide film protecting stainless steel against corrosion’, Nature Partner Journals, 7 August 2019 (Image and reference in the text).