Corrosion of Metals by Applied Alternating Currents

Abstract

The corrosion of mild steel in 0·05 M NaCl at 30°c under the influence of an applied square wave a.c. signal was studied. A mechanism for a.c. corrosion is presented, stressing the importance of reactions during a preceding cathodic half-cycle to the reactions during an anodic half-cycle and vice versa.

The corrosion rates of copper, lead and zinc under applied a.c. were also determined for comparative purposes.

It has been established that for 50 Hz sine wave a.c. applied to mild steel a peak current density of 471 mA.cm^?2 must be exceeded before serious corrosion sets in. This has been verified by a comparison of the corrosion rates as encountered in two electrode boilers operating at current densities above and below this critical value.     

Corrosion of Mechanical Joints in Aluminium and Aluminium-Steel Structures

Abstract

This paper is primarlly concerned with the problem of poor performanee of mechanical joints, the main reason for which is inadequacy of published informaiion on joint design, construction procedure,, and maintenance. This contention is illustrated by discussion of corrosion problems that arise with mechanical joints, particularly with aluminium-stell joints which are commonly used in shipbuilding.

The need for closer co-operation amongst designers, constructors and corrosion specialists is strongly emphasised. Some methods of making practical corrosion informaiion available to industry are suggested.     

Corrosion performance of titanium and titanium stabilised stainless steels

Abstract

Titanium and its alloys provide industry with a number of materials which are strong, light, and very corrosion resistant. In addition, titanium is added as an alloying constituent to some stainless steels to act as a stabiliser during welding. Over the past 30 years titanium alloys have been increasingly used in process industries, and wherever ‘nil corrosion’ is considered to be an essential design feature. The main drawback to titanium usage has been relatively high cost, but freedom from plant corrosion failures, reduced downtime for maintenance, and the increasing availability of titanium have made this metal and its alloys an attractive choice in recent years. Applications include process vessels, heat exchangers, marine fittings, offshore components, pump castings, and other applications where materials encounter a hostile service environment. Nevertheless, titanium and its alloys are still subject to some forms of corrosiveattack, such as galvanic corrosion, hydrogen absorption, erosion corrosion, and crevicecorrosion. Special welding procedures are also required, which, if ignored, can lead toserious problems. This paper outlines a number of recent investigations into some problems encountered in industrial and marine environments, where both titanium metal and titanium stabilised stainless steels have suffered unexpected corrosion attack. The case histories described illustrate that titanium may show unexpected corrosion problems if certain aspects of its corrosion behaviour are overlooked.     

Corrosion of Aluminium—Literature Review 1965–1967

Abstract

Lewis and others report that stress-corrosion cracking and intercrystalline corrosion are the main problems associated with the aluminium alloys used in the aircraft industry. Stress-corrosion cracking occurs principally in the joints of integrally machined sections, and in forgings, as a result of the combination of stresses such as residual heat treatment stresses, airborne versus ground stresses and assembly stresses together with any corrosive environment combined with inadequate protection; in some susceptible alloys the stress corrosion takes the form of end grain attack at the radii of integrally machined sections. Intercrystaliine corrosion has also been a major problem both in the aircraft industry as well as industry in general. In the latter case the corrosion is sometimes attributed to misapplication and lack of care during handling or pretreatment.     

Localised Corrosion of Stainless Steels During Food Processing

Abstract

The austenitic stainless steels are widely used in the food industry as constructional materials for processing plant. One of their drawbacks is a susceptibility to localised attack, particularly in the presence of the chloride anion. The various forms of localised attack which stainless steels can suffer, including pitting, crevice attack, deposit attack, stress corrosion cracking and corrosion fatigue, are illustrated with case histories drawn from the food processing industry.     

Corrosion of Condenser Tubes Due to Carbonaceous Film on the Surface

Abstract

This paper describes pitting corrosion that has occurred in the condenser tubes in a numbu of power stations in the Republic of South Africa. The power stations concerned are situated inland and use fresh surface water, concentrated in cooling towers, for condenser cooling. Investigations indicate that the corrosion is similar to the pitting of copper potable water tubes caused in the U.K. and elsewhere by the presence of carbon film formed on the surface during tube manufacture. Many turbine and condenser tube manufacturers, however, disagree with these findings and some of their views are given in this paper. The author is of the opinion that accelerated corrosion due to this cause is more general than most corrosion engineers believe.     

Recognition of Corrosion Hazards to Metals in Building

Abstract

This paper outlines some of the conditions which affect durability at discontinuities in metals used in the building industry. The application of metals for lightweight cladding and the specific problems arising at crevices, dissimilar metal contacts, in run-off waters and chemical solutions and by stress corrosion are briefly considered. While individual problems of corrosion are not generally costly to rectify, collectively they Impose a heavy maintenance burden on the building industry. In this context it is argued that the faults Which occur could have been prevented by due recognition of the corrosion hazards.     

Protection of Steel Bridges From Corrosion

Abstract

Atmospheric corrosion is only one of many causes of deterioration of structures, and protective measures applied to bridges are usually successful, in the sense that disasters resulting from corrosion are rare. Most bridge steelwork requires careful protection and regular inspection and maintenance, though in certain circumstances various grades of steel can be left uncoated. Depending on the intended life of a bridge, the procedure which in the long run is the most economic is usually one which employs a high standard of initial protection, so that unnecessarily frequent repainting is avoided. The circumstances in which steel piles can safely be used to support a bridge structure are briefly discussed. The protection of superstructure steelwork begins at the design stage, when much can be done to minimise subsequent corrosion troubles. Guidance is given on the choice of protective films, with or without a preliminary coating of non-ferrous metal. The standard of surface preparation needed for each type is given and its importance emphasised. The advantages of carrying out part of the protective work at the factory are explained, and special points to be considered at joints, bearings and road decks are described.

Maintenance painting is needed between 10 and 20 times during the life of a steel bridge, and calls for careful attention backed up by regular, skilled and painstaking inspection. A good system of testing is needed, and simple tests giving quick results have distinct advantages, especially if they can be carried out at the site where painting is being done. Some actual examples of protective measures adopted on a variety of modern steel bridges are described.     

Practical Corrosion Problems in Relation to Welded Joints. II. Corrosion Problems in Design

Abstract

This paper deals firstly with the application of protective coatings to welded structures. It then discusses the economic maintenance problems involved, the welding of hollow members, stress corrosion craking and limited life design, and corrosion fatigue. The points made are illustrated by some experimental results.     

Corrosion Control at Welded Joints in Constructional Steels

Abstract

Corrosion problems at welded joints are considered. Means of corrosion control so that welds in structures have corrosion resistance equal to that of the joined members are discussed. Cheaper and easier expedients, where the technically best method is not practicable, are pointed out.     

Corrosion of Zinc—Literature Review 1965—1967

Abstract

The corrosion reactions of zinc are complicated even in apparently simple conditions, and Biestek & Niemiec have identified many new compounds in the corrosion products, while Markovic et al. and Grauer & Feitknecht have studied the thermodynamic aspects of its aqueous corrosion. Wiederholt has comprehensively reviewed the corrosion data for zinc in aqueous solution, and in more recent work by Schikorr, nitrate and carbonic acid are both shown to cause the potential of zinc to change to a more ‘noble’ value.     

Corrosion behaviour of nitrogen implanted titanium in simulated body fluid

Abstract

Titanium is one of the most important materials for medical applications, as a result of its uniquely high biocompatibility. The effect of nitrogen implantation on the biocompatibility and the corrosion resistance of cp titanium are reported. Grazing incidence X-ray diffraction studies showed that implantation formed a δ-TiN_x phase. Electrochemical tests in HBSS showed an optimal decrease in corrosion current density for specimens implanted with 3 × 10¹⁷ ions cm^-2 at 25 keV, compared with unimplanted titanium. Following implantation and immersion in a commercial physiological solution, phases are precipitated which are rich in calcium and phosphorus, and these hydroxyapatite precursors indicate that this implantation regime confers optimal properties of corrosion resistance and biocompatibility.     

Centre for Protection Against Corrosion

Abstract

A graphical method is described for calculating the instantaneous corrosion rate and anodic and cathodic. Tafel slopes from the cathodic polarisation curve. Unlike other methods, which use a portion only of the polarisation curve, the method suggested uses the cathodic polarisation as a whole. Examples have been given to show that this method can be used to obtain the corrosion parameters readily, and with reasonable accuracy.     

Influence of Surface Condition on the Corrosion of Aluminium Brass

Abstract

The corrosion behaviour of aluminium brass, with four different surface treatments (highly and lightly oxidised, pickled and sandblasted), was studied in natural sea water, both stagnant (aerated) and flowing.

Electrochemical measurements and weight loss determinations showed that the oxide film produced during industrial annealing had little effect on the corrosion rate as compared with a pickled surface, in both stagnant and flowing conditions. Sandblasted surfaces were inferior.

Measurements of currents flowing in galvanic couples formed between oxidised and stripped metal were small and decreased with time, but the oxidised surfaces appeared to be slightly more vulnerable than the pickled ones to localised corrosion. Some further work is needed here.     

Corrosion performance of chemically polymerized poly(pyrrole-co-o-toluidine) coating on mild steel

The Poly(pyrrole-co-o-toluidine), (PY/OT) copolymer was prepared by chemically oxidative polymerization of pyrrole (Py) and o-toluidine (OT) in HCl aqueous medium. The copolymer was characterised by using Fourier Transform Infrared Spectroscopy (FTIR) technique. The coating of PY/OT copolymer on 8-Hydroxyquinoline sulphonic acid (HQSA) pretreated mild steel surface was chemically deposited using N-methyl-2-pyrrolidone (NMP) as the solvent. The corrosion performance of the PY/OT copolymer coating on steel, both scribed and unscribed samples, was studied by conducting immersion tests and electrochemical tests which include free corrosion potential measurements and potentiodynamic polarization measurements. The above tests were conducted in different corrosive media which include 0.1 M HCl, 5% NaCl solution, artificial seawater and distilled water. The performance of coating in open atmosphere was also evaluated by conducting atmospheric exposure test. The results of different studies indicate that PY/OT copolymer coating may be considered as potential coating material for corrosion protection of mild steel in the above corrosive medium.     

Corrosion performance of friction surfaced nickel aluminium bronze (NAB) alloy under erosion corrosion and salt fog environment

ABSTRACT

In the present research work, the extruded nickel aluminium bronze (NAB) alloy is friction surfaced over cast NAB alloy as a repairing technique for the deteriorated marine components. The friction surfaced specimens were tested for their corrosion properties. The corrosion tests such as salt fog test, erosion–corrosion test and pitting corrosion test were conducted to investigate the corrosion behaviour of the friction surfaced NAB alloy. It is found that the accumulation of corrosion occurred due to the presence of a continuous network of the ΚIII-phase. The lower corrosion rate is achieved in friction surfaced NAB alloy compared to the base NAB alloy which is contributed by the corrosion resistant contents such as Al and Cu and also by the fine grain microstructure obtained during the friction surfacing process.

This paper is part of a supplementary issue from the 17th Asia-Pacific Corrosion Control Conference (APCCC-17).     

Improved Method for Measuring Corrosion Rate with Dual Electrodes

Abstract

General equations for dual electrode polarisation have been derived in this work. The equations suggest an improved method for measuring the corrosion rate giving less error than previous methods. In the proposed method, forward and reverse currents are applied to obtain the same increment and decrement of potential, respectively. The average of the forward and reverse currents is used to obtain the mean corrosion current of the two electrodes. The errors corresponding to forward and reverse currents compensate one another, and the corrosion rate calculated from the average of the two has less net error than that calculated by Barnartt's method in which only the forward current is used. The proposed method gives relatively low error independent of the corrosion potential difference between the two electrodes and overcomes the inherent weakness of Marsh's method in which errors vary with the corrosion potential difference.     

Control of Corrosion at Mechanical Joints

Abstract

Control is defined as the reduction of corrosion risks to an acceptable level by methods incurring an acceptable cost. Choice of fasteners and protective finishes for fasteners are considered and methods of preventing corrosion at joints in steel, aluminium and non-metallic structuresare discussed. It is emphasised that the fastener must be considered in relation to the other components of the joint and to the condition of service.     

Mechanism of Pitting Corrosion of Copper in Supply Waters

Abstract

Detailed examination of a large number of copper water tanks and pipes showing pitting corrosion (taken from certaind is districts to which pitting is confined) revealed features that could not be reconciled with the conventiona1 explanation based on the presence of large cathodic areas surrounding small anodic corrosion Sites. A new theory of pitting corrosion of copper is proposed which explains the features observed Pitting arises when a pocket of cuprous chloride forms beneath a porous electrically conducting membrane (usually cuprous oxide). The anodic and cathodic processes occur on the inner andouter surfaces of the memberane. Dissolution of copper occurs by reaction with the anodic product beneath the membrane and deposition of calcium carbonate occurs by reaction of hardness salts in the water with the cathodic product above the membrane.

Pockets of cuprous chloride capable of developing into corrosion pits can only form if the potential of the copper rises above 90mV EH during the period of film formation. Control of the potential below this value by coupling to a small sacrificial aluminium anode has been shown to prevent formation of pits in copper tanks and cylinders in service.     

Dithiocarbamates,Carbon Disulphide and the Corrosion of Tinplate

Abstract

The fungicides TMTD (tetramethylthiuram disulphide) and ziram (zinc dimethyldithiocarbamate) decompose to carbon disulphide and dimethylamine in plain tinplate cans of acid foods. Trace levels of carbon disulphide cause pitting corrosion of the base steel of tinplate with little or no attack on the tin coating. Pitting corrosion differs from the usual form of attack in acid media in which tin is preferentially dissolved and steel is cathodically protected. Carbon disulphide induces pitting corrosion by reducing the hydrogen overvoltage of the tin coating so that cathodic protection of the steel becomes less effective. Carbon disulphide also accelerates anodic corrosion of steel. These effects are of sufficient size to cause inversion of the tin/steel couple potential. Dimethylamine increases hydrogen overpotential of tin slightly but has negligible effect on corrosion.