Neutral solutions corrosion

Tamura, H., K. Goto, and M. Nagayama (1976), The Effect of Ferric Hydroxide on the Oxygenation of Ferrous Ions in Neutral Solutions", Corrosion Sci. 26,197-207. 

Tamura H, Goto K, Nagayama Y. 1976. The effect of ferric hydroxide on the oxygenation of ferrous iron in neutral solution. Corrosion Science 16 197-207. 

Cohen, M. 1976. The Breakdown and Repair of Inhibitive Films in Neutral Solution. Corrosion, 32,12. 

In the case of a neutral solution (e.g. pH = 7), depending on the corrosion potential all these tliree ranges (stability, dissolution or oxide fonnation) may be involved. 

There are no films or protective surface films on active metals, e.g., mild steel in acid or saline solutions. Passive metals are protected by dense, less readily soluble surface films (see Section 2.3.1.2). These include, for example, high-alloy Cr steels and NiCr alloys as well as A1 and Ti in neutral solutions. Selective corrosion of alloys is largely a result of local concentration differences of alloying elements which are important for corrosion resistance e.g., Cr [4],... 

This reaction occurs with overall cathodic currents, i.e., with cathodic polarization. It can be practically ignored in the case of free corrosion of steel in a neutral solution. Other oxidizing media are of interest only in special cases. 

Reactions with aqueous solutions. Uniform dissolution or corrosion of metals in acid, alkaline or neutral solutions (e.g. dissolution of zinc in hydrochloric acid or in caustic soda solution general corrosion of zinc in water or during atmospheric exposure). Reactions with non-aqueous solution (e.g. dissolution of copper in a solution of ammonium acetate and bromine in alcohol). 

The region of immunity [Fig. 1.15 (bottom)] illustrates how corrosion may be controlled by lowering the potential of the metal, and this zone provides the thermodynamic explanation of the important practical method of cathodic protection (Section 11.1). In the case of iron in near-neutral solutions the potential E = —0-62 V for immunity corresponds approximately with the practical criterion adopted for cathodically protecting the metal in most environments, i.e. —0-52 to —0-62V (vs. S.H.E.). It should be observed, however, that the diagram provides no information on the rate of charge transfer (the current) required to depress the potential into the region of immunity, which is the same (< —0-62 V) at all values of pH below 9-8. Consideration of curve//for the Hj/HjO equilibrium shows that as the pH... 

The Al-HjO diagram does show, however, the danger that may arise due to an increase in pH when the metal is cathodically protected in near-neutral solutions indeed, the possibility of alkaline corrosion has seriously limited the use of cathodic protection for aluminium structures. 

From these two examples, which as will be seen subsequently, present a very oversimplified picture of the actual situation, it is evident that macroheterogeneities can lead to localised attack by forming a large cathode/small anode corrosion cell. For localised attack to proceed, an ample and continuous supply of the electron acceptor (dissolved oxygen in the example, but other species such as the ion and Cu can act in a similar manner) must be present at the cathode surface, and the anodic reaction must not be stifled by the formation of protective films of corrosion products. In general, localised attack is more prevalent in near-neutral solutions in which dissolved oxygen is the cathode reactant thus in a strongly acid solution the millscale would be removed by reductive dissolution see Section 11.2) and attack would become uniform. 

Most cases of crevice corrosion take place in near-neutral solutions in which dissolved oxygen is the cathode reactant, but in the case of copper and copper alloys crevice corrosion can occur owing to differences in the concentration of Cu ions however, in the latter the mechanism appears to be different, since attack takes place at the exposed surface close to the crevice and not within the crevice in fact, the inside of the crevice may actually be cathodic and copper deposition is sometimes observed, particularly in the Cu-Ni alloys. Similar considerations apply in acid solutions in which the hydrogen ion is the cathode reactant, and again attack occurs at the exposed surface close to the crevice. 

It is also of interest to note that Wranglen considers that the decrease in the corrosion rate of steel in the atmosphere and the pitting rate in acid and neutral solution brought about by small alloying additions of copper is due to the formation of CU2S, which reduces the activity of the HS and Scions to a very low value so that they do not catalyse anodic dissolution, and a similar mechanism was put forward by Fyfe etal. to explain the corrosion resistance of copper-containing steels when exposed to industrial atmospheres. 

Previous considerations of pitting have been largely confined to metals and alloys that have a strong tendency to passivate, but since the pitting of copper has a number of unusual features it is appropriate to consider it in some detail. Reference to the potential-pH diagram for the Cu-H O (Section 4.2) system shows that in neutral solutions at the potentials encountered in oxygenated waters the stable form of copper is Cu O, and the corrosion resistance of copper thus depends upon whether or not the CU2O forms a protective film. 

The dissolution of passive films, and hence the corrosion rate, is controlled by a chemical activation step. In contrast to the enhancement of the rate of dissolution by OH ions under film-free conditions, the rate of dissolution of the passive film is increased by increasing the ion concentration, and the rate of corrosion in film-forming conditions such as near-neutral solutions follows the empirical Freundlich adsorption isotherm ... 

Oxygen Dissolved oxygen is probably the most significant constituent affecting corrosion, its importance lying in the fact that it is the most important cathodic depolariser in neutral solutions. Other depolarisers also occur, but as oxygen is an almost universal constituent of natural waters its importance will readily be understood. 

If oxidising salts are present in neutral solutions they may reduce corrosion of the iron by the establishment of thin protective films on the metal surface. Their effectiveness is considerably diminished if much chloride ion is also present in solution. 

Salts giving an alkaline reaction may be corrosive to the irons, and while neutral solutions can be handled safely there is usually little point in using high-silicon irons for these relatively innocuous solutions. The irons are useful in handling acidic solutions, subject to the restrictions already referred to regarding the halide, sulphite and phosphate ions. 

The standard electrode trotential, Ep, 2+ Pb = —Q.126V . shows that lead is thermodynamically unstable in acid solutions but stable in neutral. solutions. The exchange current for the hydrogen evolution reaction on lead is very small (-10 - 10"" Acm ), but control of corrosion is usually due to mechanical passivation of the local anodes of the corrosion cells as the majority of lead salts are insoluble and frequently form protective films or coatings. 

Electrochemical aspects of the stress-corrosion behaviour have been investigated, mainly in neutral solutions. The open-circuit potential of Ti-8Al-lMo-l V is —800mV (v5. S.C.E.). The crack initiation load reaches... 

Fig. 10.6 Polarisation diagram showing the limited role hydrogen evolution plays at the corrosion potential of steel in aerated neutral solution, the larger role in determining cathodic protection currents and the dominant role in contributing to current requirements at very negative potenitals. The dotted line shows the total cathodic current due to oxygen reduction and...

Although halide ions are aggressive in near-neutral solutions they can be used to improve the action of inhibitors in acid corrosion (see Practice Acid Solutions). Variations exist among the halides, e.g. chloride ions favour the stress-corrosion cracking of Ti in methanol whereas iodide ions have an inhibitive action ... 

The mechanisms of corrosion inhibition will be described separately for acid and neutral solutions, since there are considerable differences in mechanisms between these two media. Definitions and classifications of inhibitors are given in Section 17.2 and by Fischer. ... 

Little work has been carried out on the mechanism of inhibition of the corrosion. of copper in neutral solutions by anions. Inhibition occurs in solutions containing chromate , benzoate or nitrite ions. Chloride ions and sulphide ions act aggressively. There is evidence that chloride ions can be taken up into the cuprous oxide film on copper to replace oxide ions and create cuprous ion vacancies which permit easier diffusion of cuprous ions through the film, thus increasing the corrosion rate. 

The mechanism of action of inhibitive anions on the corrosion of iron, zinc and aluminium in near-neutral solution involves the following important functions ... 

Mechanistically, in approximately neutral solutions, solid state diffusion is dominant. At higher or lower pH values, iron becomes increasingly soluble and the corrosion rate increases with the kinetics approaching linearity, ultimately being limited by the rate of diffusion of iron species through the pores in the oxide layer. In more concentrated solutions, e.g. pH values of less than 3 or greater than 12 (relative to 25°C) the oxide becomes detached from the metal and therefore unprotective . It may be noted that similar Arrhenius factors have been found at 75 C to those given by extrapolation of Potter and Mann s data from 300°C. 

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