Terne coatings

Due to concerns about the effects of lead in the environment, work is now under way to find suitable replacement materials. Zinc alloy coatings (hot dip zinc-iron or electroplated zinc-nickel) with an organic topcoat are among the most likely candidates. 

Nickel-terne-coated steel includes an electrolytic flash coating of nickel (1-1.5 g/m ) underneath a conventional lead-tin coating for enhanced corrosion resistance. Applications are similar to the conventional lead-tin alloy coatings described previously. 

---

Some of the principles of hot dipping are common to galvanising, tinning, terne coating and aluminising. 

Both potentiostatic and galvanostatic deposition techniques are used, though the latter is often preferred, because the galvanostatic mode implies that the deposition rate is almost constant, thns resulting in uniform films. Modulated current and pnlses may be useful to build up and dislodge concentrated solutions from the electrodes and hence help in confining micropat-terned coatings. 

Lead has a good general resistance to various atmospheres. As a coating, it has had its greatest application in the production of terne-plate, which is used as a roofing, cornicing, and spouting material. 

Tin coatings are widely used in the electrical industry because of their good contact properties and in the food industry because of low toxicity. In addition to pure tin coatings a number of alloy coatings have been developed for special applications, e.g. tin-lead (terne plate), tin-zinc, tin-cadmium, tin-bronze and tin-nickel. Reference should be made to Section 13.5 and to the publication by Britton for data on the corrosion of tin and its alloys. 

These alloy coatings have advantages over tin in atmospheric exposure where there is heavy pollution by oxides of sulphur. They are cathodic to steel and anodic to copper. In industrial atmospheres, however, formation of a layer of lead sulphate seals pores and produces a generally stable surface and terne-plate has been used extensively as roofing sheet, especially in the USA. It is easily and effectively painted when additional protection is required. Copper heat exchangers in gas-fired water-heaters may be coated by hot dipping in 20% tin alloy . 

Tin-lead can be substituted for tin for other purposes, although the toxicity of lead limits the field of application. The corrosion resistance is usually no better than that of unalloyed tin, but there may be some saving of cost in applications such as wash-boilers and other vessels for non-potable liquids and light engineering components formed from sheet metal. Heavily coated terne-plates may be used for the fuel tanks of stoves and vehicles. 

Terne Plate. This is a sheet-steel product that is coated with an alloy of tin and lead. The coatings range from 50-50 mixtures of lead and tin to as low as 12% dn and 88% lead. Plate used for roofing normally is about 25% tin and 75% lead. In addition to roofing, terne plate is used in the manufacture of gasoline tanks for automotive vehicles, oil cans, and containers for solvents, resins, etc. 

Figure 22. Exterior surface with pat- Figure 23. Spherical void in the cross terned cracks in an improperly cooled section of a pressure-atomized sulfur sulfur coating coating...

The values of T and Ro for a layer of thickness X are expressed in terns of an absorption coefficient K and a scattering coefficient S. The values of K and S that should be used are the respective suns of the absorption and scattering coefficients of all the conponents in the coating, not just the photo-initiator. 

Coat the following components with the proper anti-high tern perature grease ... 

Conversion. The cure of many oxidative thermosetting coatings is accelerated by heating. In other resin systems, such as thermosetting acrylics and alkyd melamines, the reactions do not occur below tern-... 

---