Corrosion Resistance Data

Introduction

The results of a survey of users’ attitudes to brass showed that ‘Corrosion Resistance’ was the most highly appreciated property. In average industrial, commercial and domestic environments, brass lasts well and is fit for purpose for many years, showing only a superficial darkening with age. In use in more aggressive environments, careful consideration has to be given to product design, material selection and finishing. This is true for all materials. The following comments explain some of the corrosion mechanisms that can be encountered in extreme environments and techniques for controlling their effects.

General corrosion and tarnishing

General corrosion and tarnishing are probably the manifestations of corrosion most readily recognised by the man in the street. Typical examples are rusting of steel, the development of brown tarnish (and, under more severe exposure conditions, a layer of green corrosion product) on copper, and the widespread formation of small corrosion pits on unprotected aluminium - especially under semi-sheltered exposure conditions such as the underside of bus shelter roofs. Brasses show considerably greater tarnish resistance then copper with no tendency to severe general attack comparable to rusting or to significant pitting.

A domestic example which everyone takes for granted is the pins of electric plugs which remain free from corrosion, other than very slight tarnishing, almost indefinitely in indoor service. This safety-critical product remains reliable for many years.

In outdoor exposure conditions, especially where there is industrial pollution of the atmosphere or in situations very close to the sea, a heavier tarnish develops on most brasses. This eventually produces a thin deposit of brown-green copper compounds which, since it is adherent and spreads uniformly across the surface, helps to protect against further attack. Consequently, unless aesthetic considerations require the preservation of the original appearance of the brass no protection is generally necessary. If it is desired to retain a bright appearance this may be achieved by regular cleaning or by lacquering.

Suitable lacquers for different conditions of service are discussed in Section 4.

The influence of zinc content

Because of their higher zinc content, alpha-beta brasses generally show better tarnish resistance than alpha brasses under mild or moderately severe exposure conditions but, under more severe conditions may be affected by dezincification corrosion as described later. The influence of zinc content is illustrated by the results in Table 27.

These data show, in addition to the effect of zinc content, a progressive reduction in corrosion rate with time for all alloys and a greater loss in strength, relative to corrosion rate calculated from weight loss, for the alpha-beta brasses than for alpha brasses. Table 27 compares seven-year exposure results for rural, marine and urban sites in Sweden.

Table 27 Average penetration of Copper-zinc alloys in seven-year atmospheric exposure tests ()

(1) Average for 7 year period, calculated from weight loss. (1m m = 0.0004in)

The influence of other alloying additions

The nickel silvers are more resistant to tarnishing than ordinary brasses, the least tendency to tarnishing being shown by those of highest nickel content. Under indoor exposure conditions the tarnishing results only in the development of a yellow tinge in place of the original silvery appearance but long-term outdoor exposure can produce darker surface staining and, eventually, a deposit of light green corrosion products.

Aluminium also confers increased tarnish resistance. Aluminium brass consequently retains its original appearance much longer than other alpha brasses though, in common with other brasses containing arsenic, it eventually develops a blackish tarnish rather than the usual brown.

Staining during transport or storage

An occasional problem with brass semi-finished products, such as sheet, is the development of patchy brown areas of tarnish during transport or storage. This staining is sometimes due to sulphide but more often simply to rain or condensed water drawn in between the brass sheets in a pack, or between adjacent turns in a coil. The unusual conditions of water retention in the narrow gap give rise to staining which would not occur under normal fully exposed conditions. For many manufacturing purposes the presence of slight water or sulphide staining on brass sheet or strip stock is not important since it does not represent any significant damage except for its effect on appearance. It is, however, obviously sensible not to leave sheet or strip stock unnecessarily exposed to the weather. Manufacturers often take additional precautions to prevent staining in transit and storage by the use of the inhibitor benzotriazole (bta), either by direct application to the metal or by interleaving with bta-impregnated paper. Methods of using bta to inhibit tarnishing and staining of brass products are discussed in Section 5.

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