Zinccoated Steel

The zinc coating of steel has for many years been a standard method for its protection against corrosion. The zinc coating may be applied by hot-dipping or spraying with the molten metal, sheradizing in heated

Fig. 5.16 Stainless steel construction - Lloyd's Building, London. Architects: Richard Rogers. Photograph: Arthur Lyons
Table 5.8 Stainless steel compositions and grades to BS EN 10088-1: 2005 for different environmental conditions

Designation

Suitable environments

Type

Name (indicating composition of

Number

alloying components)

Austenitic

X5CrNi18-10

1.4301

rural and clean urban

X5CrNiMo17—12—2

1.4401

urban, industrial and marine

Ferritic

X6Cr17

1.4016

interior

Duplex

X2CrNiMoN22—5—3

1.4462

severe industrial and marine

Notes:

Cr, Ni, Mo and N refer to chromium, nickel, molybdenum and nitrogen respectively. X2, X5 and X6 refer to the carbon contents of 0.02, 0.05 and 0.06% respectively.

Notes:

Cr, Ni, Mo and N refer to chromium, nickel, molybdenum and nitrogen respectively. X2, X5 and X6 refer to the carbon contents of 0.02, 0.05 and 0.06% respectively.

zinc powder or electrodeposition. In hot-dip galvanising the steel is cleaned by pickling in acid followed by immersion in molten zinc or iron-zinc alloy. The zinc coating protects the steel by acting as a physical barrier between the steel and its environment, and also by sacrificially protecting the steel where it is exposed by cutting or surface damage. The iron-zinc alloy coating gives a better surface for painting or welding.

The durability of the coated steel is dependent upon the thickness of the coating (standard 275 g/m2, i.e. 137.5 g/m2 per face) and the environment. Coastal situations and industrial environments with high concentrations of salt and sulfur dioxide respectively may cause rapid deterioration. The alkalis in wet cement, mortar and plaster etch zinc coatings, but once dry, corrosion is slow; however, calcium chloride used as an accelerator in plaster is aggressive and should only be used sparingly. Fixings for zinc-coated sheet should be carefully chosen to avoid the formation of bimetallic couples, which can cause accelerated corrosion. In particular, no copper or brass should make contact with either zinc or iron-zinc alloy coated steel. Other metals such as lead, aluminium and stainless steel have less serious effects in clean atmospheres, but generally all fixings should be sealed and insulated by rubber-faced washers. Where zinc-coated steel is to be fixed to unseasoned timber or timber impregnated with copper-based preservatives, the wood should be coated with bitumen paint. Where damaged in cutting, fixing or welding, the zinc coating should be repaired with the application of zinc-rich paint.

Zinc-coated steel may be painted for decoration or improved corrosion resistance. However, the normal spangle zinc finish will show through paint and the minimised spangle or iron-zinc alloy finish is more appropriate for subsequent painting.

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