Coatings for marine environments

In India the cost of corrosion prevention in marine industries may be of the order of Rs.100 crores. The different areas of service are: (1) Immersed in seawater (2)'Splash waterline (3) Super structure area near seawater. Painting is the principal means of protection; other methods such as cathodic protection, are used only in conjunction with paint coatings. Hence it is necessary to provide long term maintenance-free protection and to minimise the time out of service when maintenance of ship is essentially required. In this article the mechanism of corrosion prevention by paints, pretreatment, paint system, method of application of paint for &fferent types of structures and equipments used in marine environments are described. A comprehensive protective system consisting of primer + under coat + finishing paint + antifouling paints is included. Application of the organic coatings and details of type of fouling and the use of different types of antifouling compounds used in the paint formulation are given. The protective schemes for dock and harbour installation, offshore structure, super structures and deck areas are discussed. The causes of paint failure and the specification, requirements of paints are described

Electrodeposition of zinc-manganese alloy from sulphate-citrate bath

Zinc-manganese alloy has better corrosion resistance than the individual metals. Hence, in place of pure zinc, zinc-manganese alloy coatings can be used to protect steel against corrosion. It is reported that zinc-manganese alloy containing 0-90% manganese could be electrodeposited from the sulphate-citrate bath. Most of the work reported are patented. Hence, in this paper the authors have made an attempt to deposit zinc-manganese alloy from a sulfate-citrate bath and studied in detail the bath development, alloy composition, cathode efficiency and polarisatio

Electrochemical behaviour of titanium supported Ir02/Zr02 electrodes in acid and alkaline solutions and in ferricyanide/ferrocyanide redox systems

Mixed oxides of IrOz and ZrOz were prepared in various mole ratios and their cyclic voltammetric behaviour, and Tafel behaviour for oxygen evolution reaction in acid and alkaline solutions were studied. The service life of these electrodes were evaluated in 6 mol/dm3 NaOH solutions and it is found that the electrodes containing 60 mole ,% of lrOz showed a maximum service life. The electrochemical behaviour of these electrodes were also studied in ferricyanidelferrocyanide redox systems

Physical and corrosion resistant properties of rapidly quenched lead-antimony and lead-antimony-tin alloys

Molten lead-antimony and lead-antimony-tin alloys were spiat quenched in the spinning wheel sel up under overhead pressure of 703 g/sq.cm. for preparation of ribbons. Physical properties of ribbons such as density, microhardness, thickness, electrical resistance and melting point were determined. The corrosion resislance properties of the ribbon and bulk alloy were delermined by weight loss after immersion in corrosive electrolyte. The potenlial-lime and galvanoslatic measurements of these ribbons in 3 % NaCl solulions were carried ou

Fused salt plating of lead-cadmium alloy on mild steel

The electrolytic deposition of lead - cadmium alloy was carried out on mild steel substrate, using fused salt bath, such as sodium nitrite, containing lead nitrate and cadmium nitrate. The influence of electrolyte composition, current density and time to get satisfactory deposit has been investigated. The physical and corrosion resistant properties of the coating were studied and reported in this paper

Evaluation of protective schemes by capacitance and resistance measurements

Capacitance measurements as a function of time can give specific information on the uptake of water by paint Nms. More recently it has bcm shown that information on dielectric behadour of paint before and after immersion in corrosive medium can be obtained by the measurement of capacitance as a function of frequency. The resistance and capacitance values of different indigenously prepared paint schemes were compared with imported schemes. The results are discussad h relation to the water u p take of the paint fdms

Electrodeposition of modified cnsl resin on mild steel

A method has been described for the preparation of water soluble CNSL resin using formaldehyde for use in electrodeposition. The bath composition and operating conditions to get good deposit OR mild steel was standardised. The coated panels were subjected to physical tests, accelerated tests and electrochemical tests for assessing the protective property of the coatings

Performance of electrodeposited nickel,chromium, zinc, tin-bismuth alloy coating panels exposed at mandapam camp

Bright nickel with a top layer of conventional chromium continues to be the one still in use as a decorative finish. Electrodeposited nickel plus flash chromium, zinc, and nickeliron alloy plus flash chromium coatings on mild steel and tin-bismuth alloy on brass substrates were prepared by the Metal Corrosion Protection Institute. Sofia, Bulgaria which had been exwsed at Mandapam Camp. The performance of the coatings was assessed for a period of one year. he protective schemes. Ni-Cu-Cr, ~i-~e.CNci- Cr withstood upto six months in the highly corrosive site at Mandapam Cam

Some Electrochemical Aspects of Corrosion prevention by paints, metallic coatings and inhibition

An artificial pit technique was developed to study the galvanic relationship between the substrate and coating metals for a number of systems. The systems studied are steel plated with nickel and chromium, steel coated with sprayed aluminium and steel given dip coating with aluminium and tin-M.S. The study consisted in: a) Measurement of galvanic current and potential flowing between the substrate and the coating metals by using zero resistance ammeter method and VTVM. b) Potential decay when the current is cut off These were done in several corrosive electrolytes. The most significant results obtained are given below: a) In the Fe-Ni-Cr system the presence of nickel undercoat is necessary to eliminate the large galvanic current that flow between iron and chromium unlike the small current flow between nickel and chromium. b) At pores in zinc-aluminium coated steel, steel behaves like hydrogen electrode as long as the pore diameter is small. This has been explained due to the well-known propensity of steels to be charged with hydrogen. c) Sprayed aluminium gives more negative potential than steel in organic acids and can be considered as substitute for tin coating. The results are published. A Comprehensive investigation into the mechanism of protection by primers so as to get insight into the scientific basis for the formulation of new and better primers. The whole range of chromate primers, red lead red oxide primers with different ratios, primers using CNSL and other resins were all taken up for study. The investigation consisted in: a) Analysis of aqueous extracts of pigment vehicle and paints. b) Potential and polarization studies with bare metal in aqueous extract and with painted metal in salt solution. c) Electrical resistance measurements with painted metal. d) Accelerated corrosion rates and water absorption test with painted metal. These investigations lead to the following new results: a) The solubility of pigment in aqueous medium differs with nature of the vehicle employed. b) Extenders can modify the amount of pigment extracted in aqueous solutions. c) The inhibitive character of primers can be attributed to different reasons such as solubility of the pigment, formation of inhibitive soaps, formation of surface complex between the vehicle and metal surface