Corrosion Engineering,Science and Technology: The International Journal of Corrosion Processes and Corrosion Control

On-load corrosion affects the mild or low-alloy steel surfaces of boiler tubes where steam is being generated at temperatures up to 360°c, and pressures up to 180 bar. The corrosion rate under these conditions is low in pure water or dilute solutions, but is much higher in more concentrated solutions af either high or low pH. On-load corrosion results from the local formaiion of either concentrated alkaline or acidic solutions. These are generated by the concentration of dilute solutions by boiling when the heated surface is obstructed by a steam blanket, a crevice or a porous deposit.

Corrosion due to concentrated alkaline solutions was recognised first in the thirties. The alkalinity of boiler water consequently tended to be reduced and corrosion due to acidic solutions increased during the forties and fifties. The effect of acid-forming contaminants was demonstrated in the sixties and acidic corrosion due to oxygenated chloride solutions was established even more recently. Understanding the mechanism of on-load corrosion has led to the present satisfactorily low incidence of corrosion to boilers.

Prospects of changes in water treatment, design or alloy composition are briefly discussed.

The problems of impingement corrosion of condenser tube materials are reviewed, with special reference to the power industry of the UK. The South Eastern Region of the Central Electricity Generating Board has set up a laboratory to study the corrosion behaviour inflowing sea water of metals used in power station cooling water systems. The jet impingement facilities of the laboratory are described and the results of recent work presented. These include the impingement characteristics of some standard condenser tube materials, the modification of these characteristics by such methods as cathodic protection and ferrous sulphate dosing of the sea water, and the resistance to impingement corrosion of some new materials. Particular attention is paid to titanium and its resistance to this form of corrosion.

The corrosion of carbon steel in aqueous chloride solutions at different pH values Was studied at 25°c and 96°c. The influence on the corrosion rate of the steel of intermittent bubbling of (Cl₂ + N₂) and/or (H₂S + N₂) through the chloride solution was also investigated. Gases containing chlorine increased the corrosion rate and the pitting tendency, whereas gas mixtures containing H2S reduced the corrosion rate and the pitting tendency of the specimen. Pre-treatment with Cl₂ or with H₂ increased the corrosion rate. Cold working increased both the corrosion rate and the tendency to pitting. Trisodium phosphate partially inhibited the corrosion by chloride, but complete passivation was never achieved at any concentration of phosphate.

The corrosion of austenitic stainless steels types AISI 304, 310 and 316, and of Inconel alloy, was studied at 25°c, in 5% NaCl solution at an initial pH value of 2·5, and in 5% FeCl³ at pH 1·2. The resistance of the alloys in both corrosive environments was in the order: 310 > 316 > 304 > Inconel. Pre-treatment of the specimens with bubbling chlorine gas increased the subsequent corrosion rates of the alloys. Intermittent bubbling of gas mixtures such as Cl₂, N₂, and/or H₂S, increased the corrosion rate of Inconel alloy when Cl₂ was present, but decreased the corrosion rate when H₂ was present. Heat treatment of austenitic stainless steels increased the subsequent corrosion rates, whereas 16% pre-straining of annealed specimens slightly reduced the rates. Addition of trisodium phosphate to the corrosive solution reduced the corrosion rates and pitting tendency for all three types of austenitic stainless steel.

An investigation has been made into the hydrochloric acid pickling of a plain carbon X65 grade steel and a low alloy P110 steel to determine whether the treatment introduces significant quantities of hydrogen into the steels. For pickling times up to 6 h the hydrogen absorbed by both grades of steel was found to be low, although the P110 steel absorbed more than the X65 steel in a given time. Scaled samples of both grades of steel were found to absorb more hydrogen than the unsealed ones Reasons for these facts are discussed.

Experiments were made with Glasgow tap water and de-ionised water in contact with copper tubing with capillary (soldered) jointing, used for drinking water services, after the detection of contamination by lead in the water services in a large modern building in Glasgow.

It was found that the lead pick-up in a building is subject to variations. Newly constructed fittings can release 200–300 μg of lead per fitting over a 16 hour or overnight period. The release of lead decreases over 4–5 weeks to 10–30 μg per fitting. The lead released from joints removed from a building after five years of use showed a mean value of 22 μg per fitting.

A simple cell: solder/water + electrolyte/copper tube, shows that the solder is anodic and that the lead release from capillary joints is electrolytic in nature, with plumbers’ flux acting as the electrolyte.

Long-chain n-alkyl-trimethyl- and -triethyl-ammonium sulphates give good inhibition of the corrosion of mild steel in 0·5 M H₂S0₄, but little or no inhibition with pure iron. This is probably because mild steel is negatively charged in O·5 M H₂SO₄ whereas pure iron ispositive.

Although the sulphates of n-alkyl-trimethy- and -triethyl-ammonium compounds give little inhibition of the corrosion of pure iron in 0·5 M H₂SO₄, the corresponding bromides give good inhibition. The experimental results suggest that there is a mutual stimulation of adsorption between the bromide and the quaternary ammonium ions.

Polarisation measurements show that the quaternary ammonium bromides give a higher proportion of cathodic inhibition with iron than with steel, presumably because of the more restricted cathodic reaction of iron.There are considerable differences between mild steel and pure iron in the effects of the quaternary ammonium bromides on the anodic and cathodic Tafel slopes. These results are discussed in terms of the different mechanisms of adsorption of the inhibitors on steel and iron.The positive charge on pure iron enhances the specific adsorption of Br⁻ and Cl⁻ ions, and they both give appreciable corrosion inhibition in 0·5 M H₂SO₄. The effect on mild steel may be complicated by competition with HS⁻ ions, since it is found that, whereas Br⁻ ions give some inhibition, the weakly adsorbed Cl⁻ ions actually cause acceleration of corrosion.Thiourea derivatives are effective pickiing inhibitors for steel, but at low concentrations they greatly stimulate the acid attack on pure iron. At higher concentrations, above about 10⁻⁵−10⁻⁴ M, the thiourea derivatives give good inhibition of the acid corrosion of iron.

The effect of lime and silicate additives on corrosive wear of forged steel grinding balls was studied during wet grinding of a low-grade copper ore over the temperature range 25° to 65°c. Weight measurements of steel balls at various concentrations of additive in the ore slurries indicated that below pH 9·0, wear was affected by both pH and temperature. Above pH 10·5, corrosive wear was prevented. Over the temperature range studied abrasion is not greatly affected by grinding temperature. Potential measurements also indicated that the steel balls became passivated at pH values greater than 10·5 In the region of passivation, the percentage reduction of wear by lime additions was 18–23% and that by silicate additions was 20–24%. Neither additive nor grinding temperature had a significant effect on work done on ore as measured by the increase in minus 200 mesh ore during grinding.

The effect of n-decylamine (nDA) on the chemical dissolution and the anodic and cathodic polarisation of an electropolished zinc surface in an acidic chloride solution has been studied. nDA had a significant effect on the corrosion rate of zinc, maximum corrosion inhibition (90%) being observed at 10⁻³ M nDA. The weight loss measurements, and polarisation measurements, gave closely similar values for the surface coverage by nDA, the adsorption of which followed the Langmuir adsorption isotherm. nDA acts mainly as a cathodic inhibitor, but with a small degree of anodic control. The negative free energy of adsorption, and the high degree of protection, indicate the strong interaction between nDA and the zinc surface.

The tarnishing of silver and copper in iodine vapour at 30°c and at pI₂ = 62·5 Nm⁻² has been studied under an applied electric field and under short-circuited conditions. The electric field was applied across the growing iodide layer by pressing a platinum mesh over the outer surface, to act as one electrode, the metal specimen being the other electrode. Under the influence of an electrical field of either polarity, and also under short-circuited conditions, the growth for both systems, Ag–I₂ and Cu–I², conformed to a parabolic law as occurs under normal iodination.

In the Ag–I² system, increased rate of tarnishing was observed both under cathodic (specimen negative) and short-circuited conditions, compared to that under normal iodination; under anodic (specimen positive) conditions, the rate is decreased. This behaviour under the action of a field has been explained on the basis of positive hole migration. The increased rate under short-circuited conditions can be explained on the basis of cation migration through the film. An exploratory run with a Ag–4·1% Cd alloy at 100°c showed a decrease in rate under cathodic conditions. This suggests that the rate determining step changes from hole migration to ion migration, with a large increase in inherent defect concentration, as a result of the dissolution of divalent Cd in the growing iodide layer.

In the Cu–I² system, an increased rate of growth was found under cathodic, anodic and short-circuited conditions as compared with that under normal iodination. This has been explained on the basis of cation migration as in normal iodination; the parabolic growth has been assumed to be a resultant of both the mechanisms of Mott–Cabrera and of Wagner.