Corrosion Products on Metals
The study of corrosion is essentially the study of the nature of the metal reac- tion products (corrosion products) and of their influence on the reaction rate. It is evident that the behaviour of metals and alloys in most practical environments is highly dependent on the solubility, structure, thickness, adhesion, etc. of the solid metal compounds that form during a corrosion reaction. These may be formed naturally by reaction with their environment (during processing of the metal and/or during subsequent exposure) or as a result of some deliberate pretreatment process that is used to produce thicker films or to modify the nature of existing films. The importance of these solid reaction products is due to the fact that they frequently form a kinetic barrier that isolates the metal from its environment and thus controls the rate of the reaction; the protection afforded to the metal will, of course, depend on the physical and chemical properties outlined above.
In general, reaction products (films*, scales and corrosion products) may be formed under the following environmental conditions.
(a) Direct reaction with a gas (0,, CO,, CO, H, + O , , H2S, etc.) at temperatures that range from ambient to very high (1 000-2 000OC).
(b) Direct reaction with an aqueous solution with the formation of a thin invisible film (passivation) or of a thick visible corrosion product (pro- tective or non-protective).
(c) By the deliberate formation of thick oxide films (e.g. anodising) at elevated potentials or by changing the nature of existing films by chemical treatments (e.g. chromating or phosphating).
For example, in a dry atmosphere a reactive metal such as aluminium may carry a natural protective oxide film of only some 3 nm thickness, while for increased corrosion resistance aluminium may be anodised to give a coat- ing lo4 times thicker (see Section 15.1). However, thickness alone does not provide a criterion of protection; and although a thick protective layer of millscale is formed on iron and steel during processing it is not continuous owing to spalling, and the attack on the exposed substrate at the discon- tinuities is far greater than if the surface was bare. Thus the kinetics of attack
*The distinction between a film and scale is not well defined, but it is usual to use the former when referring to a thin continuous layer of reaction product (visible or invisible) whilst the latter is normally used for thick high-temperature layer (always visible).
1:22
NATURE OF FILMS, SCALES AND CORROSION PRODUCTS ON METALS 1 : 23 will be related to a variety of other factors such as composition, structure, continuity, adhesion to the substrate, cohesion, mechanical properties, etc.
of the film or scale of reaction products.
This section describes in general terms the variation in the nature of very thin films originating in the initial reaction of a metal with its environment and their progression to the thicker overgrowths that control the kinetics.
Recent developments in instrumental techniques have led to significant advances in the characterisation of these film- and scale-forming systems, and a summary of the experimental approaches available is provided at the end of the section. It is appropriate to consider first the products of reaction formed by a gaseous oxidising atmosphere and then to proceed to a con- sideration of the effect of water and aqueous systems.
Initial Surface Reaction States
The application of ultra-high vacuum techniques to low-energy electron diffraction (L.E.E.D.) studies of very clean metal surfaces in low-pressure oxidising and sulphidising atmospheres over a range of temperatures above ambient has provided detailed information on the initial states of inter- action’,’. The following sequence of events is generally observed in the case of exposure to oxygen:
1. Rapid physical adsorption of molecular oxygen.
2. Chemisorption of atomic oxygen to form a partial or complete 3. Further chemisorption of atomic oxygen into a second layer and/or In Stage 2 a distinct structural modification to an expanded lattice at sub- monolayer coverages has been observed on nickel, indicating that the oxygen ions become progressively incorporated into the metal lattice. These two- dimensional crystals then gradually transform into a three-dimensional nickel oxide lattice as more oxygen becomes incorporated. Subsequent expo- sure to high-temperature conditions (> 1 OOOOC) has confirmed the extreme stability of the Stage 2 state.
Similarly, under low-temperature conditions (< 25°C) three stages have been recognised and defined3 as follows:
1. Physical adsorption of oxygen resulting in the formation of one or more monolayers of oxide and requiring no activation energy.
2. Electron tunnelling through the stable oxide film to the adsorbed oxygen which sets up a potential and causes ion drift, thus resulting in logarithmic oxide growth.
3. Film rearrangement resulting in the formation of oxide subgrain and grain boundaries; these paths of easy ion migration promote the forma- tion of oxide ‘islands’ and result in an increase in the growth rate of the oxide.
Oxide films formed at low temperatures are initially continuous and amor- phous, but may undergo local crystallisation with the incorporation of the oxide ‘islands’, a process that is facilitated by water, heat, high electric fields and mechanical stress ‘.
monolayer.
further physical adsorption of 0,.
Thin-Film Region
Studies of thermally grown oxides in the thin-film region (e 100 nm) have revealed’ on single crystal substrates interesting details of epitaxy, stress generation, mosaic structure and film topography, and oxidation rate aniso- tropic behaviour. Mismatch between the oxide lattice and the metal substrate gives rise to stresses which may find relief in the generation of mosaic struc- tures consisting of small crystallites (5-100 nm diameter) whose lattices are slightly twisted or tilted with respect to one another. Their boundaries repre- sent potential paths of easy diffusion through the oxide.
The uniformity of film thickness is dependent upon temperature and pres- sure. The nucleation rate rises with pressure, such that at pressures above atmospheric the high rate of nucleation can lead to comparatively uniform oxide films, while increase in temperature reduces the density of oxide nuclei, and results in non-uniformity. Subsequently, lateral growth of nuclei over the surface is faster than the rate of thickening until uniform coverage is attained, when the consolidated film grows as a continuous layer2.
Growth of oxide nuclei may also be accompanied by the appearance of whiskers and platelets under certain conditions6. It has been demonstrated that oxidation of iron in air at about 200°C initially leads to nuclei of Fe,O, developing to form a porous layer. Over this homogeneous oxide layer, nuclei of a-FezO, appear and spread over the Fe,O,, but no y-FezO3 is observed. After 30 days whiskers of cy-Fez03 appear, ultimately reaching a length of 1 Fm. At higher temperatures, too, whiskers of a-FezO, appear and subsequently develop into crystallographic platelets. In general, pro- ducts of this nature occur as fine features developing from otherwise protec- tive films.
Scale-Forming Situations
In considering film growth at higher temperatures, a changeover to diffusion control, which is dependent on concentration gradients, tends to give rise to parabolic and paralinear kinetics as substantial scales form at thicknesses of 1-100 pm or more. This is the area of vital concern in the development and application of engineering alloys for high-temperature resistance, and is in distinct contrast to the thin-film rkgime. Nevertheless, the initial state of the metal surface can still influence subsequent oxidation behaviour.
Thus, different oxidation patterns may be observed depending upon whether the surface is electropolished, hydrogen reduced, mechanically abraded or cathodically pretreated. When metals of variable valency become subjected to the oxidising potential gradient across the scale, a duplex or multiple series of layers forms. The classical case of iron oxidised above 600°C has been well established6, and it has been shown that the system consists of Fe/FeO/
Fe,0,/Fez03 /Oz. In these situations film thickening occurs by transport of cations, anions, vacancies and electrons across the various phase boun- daries, which is possible owing to the non-stoichiometric composition of the various coexistent oxides (see Sections 1.8, 1.9 and 7.2).
A rather different situation arises when mild steel is exposed to liquid water or dilute sodium hydroxide at 300-360°C. Here a duplex Fe304 scale is formed, consisting of an inner adherent protective film in contact with
NATURE OF FILMS, SCALES AND CORROSION PRODUCTS ON METALS 1 : 25 an outer poorly adherent crystalline layer of magnetite (see Section 1.10).
In alloy systems the course of events is complicated by such factors'** as:
1. The affinity of the component metals for each other and for the non- 2. The diffusion rates of atoms in the alloy and of ions in the compounds.
3. The mutual solubilities of the products present in the oxidation layers.
4. The formation of ternary compounds, e.g. spinels (see Table 1.4).
5 . The relative volumes of the various phases.
In practice, thermal cycling rather than isothermal conditions more fre- quently occurs, leading to a deviation from steady state thermodynamic conditions and introducing kinetic modifications. Lattice expansion and contraction, the development of stresses and the production of voids at the alloy-oxide interface, as well as temperature-induced compositional changes, can all give rise to further complications. The resulting loss of scale adhesion and spalling may lead to breakaway oxidation9*" in which linear oxidation replaces parabolic oxidation (see Section 1.10).
Examination of the structural consequences of these complex interact- ing factors is now being elucidated in considerable detail by systematic application of electron optical and X-ray analysis techniquesg, as well as by a range of other methods''.
In certain systems the oxidation reactions may lead to a particularly pro- tective single phase being formed at the surface, e.g. magnetite (Fe,O,) in the case of iron and steel and y-Al,O, in the case of aluminium. The 'spinel' (MgAl,O,) lattice is important in relation to the protection it affords to alloys used at high temperatures, and such structures often occur with a con- tinuously varying stoichiometry as a 'double oxide' phase, which may pro- vide an effective kinetic barrier to the oxidation process (e.g. NiO*Cr,O, spinel in Cr-Ni-Fe alloys). Some examples are given in Table 1.4.
The spinel structure is of especial significance in the corrosion behaviour of iron and alloy steels both at high temperatures and in aqueous environ- ments. Its crystallographic unit cell can be represented as 8XY204 (or X, Y , 6 0 3 2 ) in which the valencies of the metal ions X and Y may be (a) XI', Y'"; (b) XI", Y'' (giving rise to the so-called '2-3 spinels' or the '4-2 spinels'; and (c) Xvl, Y'. The structure is based on a cell containing 32 oxygen atoms in a close-packed cubic arrangement. This provides for the incorporation of the X atoms in eight equivalent tetrahedral sites and the Y atoms in 16 equivalent octahedral sites. 'Inverse' spinels follow a different arrangement, represented by Y ( X Y ) O , , in which half of the Y atoms are located tetrahedrally, while the remaining Y atoms together with the X atoms are randomly arranged among the 16 octahedral positions. More generally, some spinels exist with a fraction X of Y cations in tetrahedral sites where 0 > X < f .
metal.
Table 1.4 Spinel phases encountered in alloy oxidation n-type MgFe,O,
NiFe2 0, ZnFe2 0,
ZnCr, O4 CoA12 O4 NiAI, 0,
1 : 26
It should be noted that single metal oxides such as Fe304 and c0304 are inverse spinels, while Mn304 is a normal spinel. The spinel structure is pro- minent in the oxides on iron and The oxides M,O, (and also the hydroxides and oxy-hydroxides M(OH), and MO-OH) exist in the a and y forms. Corundum and haematite represent the isostructural a forms, while they forms have cubic spinel-like structures deficient in metal ions. For example, in y-Fe,03 there are only 21 f Fe3+ ions per unit cell of 320’- ions, and these are randomly distributed among the eight tetrahedral and 16 octahedral ‘available’ sites. In magnetite, represented as Fe3+(FeZ+ Fe3+)04, one third of the cations are Fez+ and continuous interchange of electrons between Fez+ and Fe3+ ions in the 16-fold positions accounts for its extremely high electronic conductivity. Careful oxidation of Feg O4 yields y-Fe,O,, which may be converted back into Fe304 by heating in vacuo at 250°C. Because wustite (FeO) ideally has the NaC1-type structure (f.c.c.
anion lattice), with four Fez+ and four 0’- ions per unit cell, deviations from stoichiometry lead to not every octahedral site being filled in the metal deficient lattice (e.g. at 57OOC Fe,.9,0 contains cation vacancies and com- pensating Fe3+ ions). At lower temperatures disproportionation occurs:
4FeO a-Fe + Fe304
Therefore the relationship between these interconvertible structures origi- nates from a cubic anion lattice of 320’- ions in the cell. With 32 Fez+
ions in the octahedral holes stoichiometric FeO is formed. Replacement of a number of Fez+ ions with two-thirds of their number of Fe3+ ions main- tains electrical neutrality but provides non-stoichiometric Fe, - xO. Conti- nual replacement in this way to leave 24 Fe atoms in the cubic cell produces Fe304, and further exchange to an average of 21fFe3+ ions leads to y-Fe203
Fe, -,O -+ Fe304 4 y-Fe203
In actual oxidation, the cubic anion lattice becomes extended by the addi- tion of new layers of close-packed 0’- ions into which Fe atoms migrate to give rise to the appropriate stable structures.
The defect y-structures may be stabilised by the presence of Li+ or H+
ions (e.g. LiFe,O,). Cation diffusion rates in these and other lattices developed on metal surfaces play an important r61e in governing corrosion behaviour .
Surface Reaction Products Formed in Aqueous Environments
Whereas a film formed in dry air consists essentially of an anhydrous oxide and may reach a thickness of 3 nm, in the presence of water (ranging from condensed films deposited from humid atmospheres to bulk aqueous phases) further thickening occurs as partial hydration increases the electron tunnel- ling conductivity ’. Other components in contaminated atmospheres may become incorporated (e.g. H,S, SO2, CO,, Cl-), as described in Sections 2.2 and3.1.
Films may thus range from thin transparent oxides (passive films on Al, Cr, Ti and Fe-Cr alloys), or thin visible sulphides (on Cu and Ag) to thicker
NATURE OF FILMS, SCALES AND CORROSION PRODUCTS ON METALS 1 : 27 Table 1.5 Variations in the nature and thickness of the product formed on
aluminium under different conditions
Thickness (nm)
Dry air or Oz Amorphous A I 2 0 3 1-2
Humid atmosphere AlOOH + AlzO3.3H2O 50-100
Boiling water MOOR (or AI203.HzO 500-2000
Chemical conversion AlOOH + anions of solution 1oO0-so0O Anodic oxidation Amorphous + crystalline 1000-3000 Formation conditions Nature of oxide film
(barrier films) N,O, + anions of solution
'visible films, which may be compact, adherent and protective (anodic oxide films on Al and Ti, PbSO, films on Pb, etc.) or bulky, poorly adherent and non-protective (rust on steel, 'white rust' on Zn). In some cases, fairly precise limits can be placed on the nature and thickness of the products formed under different conditions, as with aluminium illustrated in Table 1.5. In other cases, the undesirable wastage of the basis metal (e.g. the rusting of steel) is of more significance than the thickness of the corrosion product, although the nature of the latter may provide information useful in inter- preting the mechanism of its formation.
Thus in industrial atmospheres the presence of FeSO, .4Hz 0 has been identified in combination with a- and y- FeO.OH, and the two latter incor- porate free water in excess of the composition Fez03 . H 2 0 . Furthermore, although some of the corrosion product may be adherent, most of it is not'2 (Sections 3.1 and 3.2).
In the fully immersed situation where the corrosion product is produced by a secondary reaction such as M 2 + + 2 H 2 0 + M(OH)z + 2H+, as in the case of iron or zinc in dilute aqueous aerated chloride solutions, the sites of the anodic and cathodic processes are separated, and widely so in the partially immersed condition. Thus OH- ions are formed at the cathode and Mzl ions at the anode, giving rise to dispersed M(OH)z where they meet and react; under these circumstances the corrosion product cannot influence the kinetics. If chloride or sulphate is present, a basic compound M,(OH),(X), may form whose range of stability will depend upon the con- centration of the anion pX and the pH of the solution; diagrams with axes pX and pH have been constructed that show the range of stability of these basic compounds. In the case of iron, the Fe(OH), formed initially is subse- quently oxidised to yellow FeO(0H) or Fe203 .HzO, or in low oxygen con- ditions black Fe, 0, is formed containing green reduced corrosion products.
Vertical surfaces allow ready detachment of the products formed, while they may settle on a horizontally corroding surface and provide some blanketing action, restraining access of oxygen to the surface. Precise identification of the products and a knowledge of the pH at their location on the surface may provide information on the conditions of formationt3.
Thin Passive Films
In considering passivity and passivation (Sections 1.4 and 1.9, the nature of the surface product (the passivating film) entering into the process between
the curve for active dissolution and that for the onset of film breakdown or oxygen evolution, assumes considerable significance.
As the system passes from the active to the passive state the initial inter- action depends on the composition of the aqueous phaseL4. An initial chemisorbed state on Fe, Cr and Ni has been postulated in which the adsorbed oxygen is abstracted from the water molecules’. This has features in common with the metal/gaseous oxygen interaction mentioned pre- viously. With increase in anodic potential a distinct ‘phase’ oxide or other film substance emerges at thicknesses of 1-4nm. Increase in the anodic potential may lead to the sequence
M - M - O H + M ( O H ) 2 + MO
monolayer multilayer phase oxide
which has been suggested for Ni in acid solutions, and Cd and Zn in alkaline solutions. On the other hand, Fe in strong H2S04 first forms a layer of FeSO, crystals, which at higher potentials is replaced by an Fe203 film, the normal product formed during anodic polarisation in dilute acid 15. In near- neutral solutions the passive film on Fe (2-6 nm thick) has been characterised as the so-called cubic oxide y-Fe,O, overlying a thin film of Fe,O, on the metal surfaceI6.
The nature of y-Fe,O, in passive films is very significant and has been reviewed in detail”. Here again a spinel structure is prominent (derived from magnetite). Its structure is considered to be cation defective with pro- tons (H+) progressively replacing Fez+ ions in the Fe304 spinel, and leading to a continuous series of solid solutions of which Fe,O, and Fe,O, are the end products. In some cases an HFe,O, composition is indicated in which some Fez+ ions have been replaced by protons. The implication of this mechanism of replacement of Fez+ ions is that water is incorporated into the passive film by a process of oxidative hydrolysis of the initial Fe, 0, substrate as the potential of the metal is progressively raised.
An important feature of such films is their low ionic conductivity that restricts cation transport through the film substance. Electronic semi- conduction, however, permits other electrode processes (oxidation of H 2 0 to 0,) to take place at the surface without further significant film growth.
At elevated anodic potentials adsorption and entry of anions, particularly chloride ions, may lead to instability and breakdown of these protective films (Sections 1.5 and 1.6).
Thick Anodic Films
Where the electronic conductivity of the film substance is low, as in the case of the ’valve’ metals (Al, Nb, Ta, Zr, Ti), an increase in anode potential gives rise to a high electric field across the passive layer. Under these circum- stances ion transport occurs and film growth continues to several hundred volts with thicknesses rising to hundreds of nanometres. At low voltages an amorphous or microcrystalline ‘barrier’ oxide is formed, which may recrystallise thermally or by the action of a high field to y-Al,O, , /3-Ta20, or TiO,, etc. A ‘mosaic’ structure has been attributed to these amorphous films” to account for their high field conduction properties. In the case of