Dezincification

Dezincification is an example of "dealloying" in which one of the constituents of an alloy is preferentially removed by corrosion. Another example is graphitisation of cast iron. Cast iron has a structure comprising essentially two components - graphite and ferrite. Corrosion causes progressive dissolution of the ferrite (iron) constituent leaving the graphite behind. The dezincification of brass is a little more complicated since the zinc and copper are not present as separate constituents but as alpha and beta solid solutions. The effect of dezincification corrosion is however similar to graphitisation in that one constituent of the alloy (zinc) is selectively removed leaving the other (copper) behind. The mechanism by which this occurs is probably different in that, instead of the zinc being selectively leached out from the brass, the zinc and copper both pass into solution together but the copper is then almost immediately redeposited in virtually the same position that it occupied originally. The result therefore is to remove the zinc as corrosion products and leave a residue of copper. Dezincified brass, like graphitised cast iron, retains the original shape and dimensions of the metal component before corrosion but in both cases, the residue is porous and has very little strength.

Dezincification was first recognised as a serious problem in 70/30 brass tubes used for ships’ condensers before about 1920. It was stated that "Condenseritis" i.e. Dezincification of condenser tubes had more effect than the Germany navy in putting HM ships out of action in the first world war. Research on the problem by G D Bengough and R May () established that dezincification could be prevented by the incorporation of about 0.03% arsenic in the 70/30 brass alloy and this addition is now standard in all alpha-brass tube specifications including admiralty brass and aluminium brass. Alpha-brass strip is not usually arsenical since it is mostly used in situations where dezincification does not occur or is not significant.

Dezincification as a problem with alpha-beta brass water fittings in some districts was first recognised in the late 1950’s. This was a type of dezincification, now termed "meringue dezincification", in which the zinc passing into solution from the brass forms very bulky hollow mounds of corrosion product which block the fitting. It attacks the beta phase preferentially but spreads at a later stage into the adjoining alpha phase. This is illustrated in the photo-micrograph, figure 1, in which the alpha and beta phase have both suffered dezincification near the surface of the metal but in the interior the attach is clearly restricted to the beta phase. Since the addition of arsenic to the alloy does not inhibit dezincification of the beta phase, arsenic additions are of no value in alpha-beta brasses.

Recognition

Dezincification may show itself as dull red spots developing on the surface of brass after long periods of exposure to urban or industrial atmospheres. These do not normally represent any significant loss of strength in the component concerned but, since they are more than simply superficial they cannot be removed by the cleaning and polishing procedures that would normally restore the brass to its original appearance.

Dezincification in water fittings, valves etc. can show itself in a variety of ways depending on the water composition and service conditions. Blockage due to meringue dezincification has already been mentioned. Other possible manifestations are seepage of water through the walls of fittings after long periods of service or leakage at valve seatings due to dezincification coupled with erosion of the soft, dezincified residue. The extreme case of damage by dezincification is actual breakage, with a dull coppery appearance to the fracture surface. Breakage is not common but can affect alpha-beta brass underground fittings (in which dezincification may be occurring from both the water side and the soil side) valve spindles, screws and "bronze-welded" joints.

Conditions for Dezincification

The possibility of spots of dezincification occurring as a result of long exposure to polluted atmospheres has already been mentioned. Service conditions that can give rise to more significant dezincification usually involve acidic or highly saline conditions. These include for example exposure to waters with a pH below 7. Such waters are not normally used for public supplies in the UK but some private supplies, mine waters and industrial rinse waters are sufficiently acidic to cause dezincification in susceptible brasses. Service in sea water or brackish water is also likely to produce dezincification in susceptible brasses as is burial in corrosive soils such as acid peat, salt marsh, waterlogged clay, or made-up ground containing cinders.

The particular form of dezincification giving rise to bulky corrosion products (meringue dezincification) is associated with waters having a high chloride to temporary hardness ratio, coupled with a high pH usually above 8.0 and often above 8.3 water compositions falling within the shaded area in figure 13 are liable to cause meringue dezincification of alpha-beta brass fittings. The boundary between the shaded and clear area is not precise and any water composition close to the boundary should be regarded as potentially liable to cause meringue dezincification. It should also be noted that waters with a composition just within the shaded zone can cause as rapid dezincification as waters with compositions well within it.

The water supplies to most parts of the UK, including almost all the major centres of population, are of compositions that do not give rise to meringue dezincification. The waters that do give trouble are certain moorland-derived supplies (but by no means all such waters) and lowland river supplies that have been treated by the lime-softening process. Water authorities in areas where water liable to cause meringue dezincification is supplied usually advise the use of dezincification resistant materials for water fittings. This advice does not however generally apply to terminal taps since the flow conditions in these are such that the hollow shells of meringue corrosion product do not build up.

Two factors that can increase the probability and rate of dezincification occurring in service are elevated temperature and coupling to a more noble metal. If brass bosses are used on copper hot water cylinders the combined effects of the high water temperature and coupling to a large area of copper can give rise to significant dezincification even in waters that normally give no trouble at all. Consequently this is one point in a domestic plumbing system where brasses are not used; the British standards covering the construction of copper water cylinders specifically require the bosses to be of dezincification-resistant materials.

Avoidance

Dezincification problems in service can be avoided by recognising in advance whether the service conditions are likely to produce dezincification and, if so, using appropriate dezincification-resistant brasses. For heat-exchanger or other tubing the question solves itself since all alpha brass tube specifications require the presence of arsenic in the alloy to inhibit dezincification. Alpha brass strip or sheet, other than aluminium brass, is not usually arsenical since it is mostly used for purposes where no significant dezincification will occur. For more corrosive conditions aluminium brass strip can be used, or one of the higher-copper brasses, with 15% or less of zinc, which are practically immune to dezincification. Nickel silvers also show high resistance to dezincification and can be an appropriate choice for some applications when this property is important.

If the manufacturing process involves hot stamping or requires free machining rod or bar, alpha beta brasses are normally used but these are susceptible to dezincification in unfavourable environments.

Research work solved this problem by producing brasses which, at the hot stamping or extrusion temperature, contain sufficient beta phase to be hot-worked satisfactorily but which can be converted by subsequent heat treatment to an all-alpha structure which is protected against dezincification by incorporating arsenic in the alloy. Such a forgeable, dezincification resistant brass has, since 1980, been included in BS2872, "copper and copper alloys, forging stock and forgings" and BS2874 "copper and copper alloys, rods and sections (other than forging stock)" under the designation "CZ132". CZ132 is a leaded brass and its machinability is comparable with the leaded duplex brass, CZ122, commonly used for production of water fittings. CZ132 rods and bars for machining are heat treated by the materials supplier to put them into the dezincification-resistant condition. CZ132 forging stock is supplied unheat-treated since it must be heat treated in the range 450-550oC after forging to ensure resistance to dezincification. This is done by the fittings manufacturer.

To retain corrosion resistance, fittings should not be reheated above the heat treatment temperature, as happens in brazing. If accidentally overheated, corrosion resistance can be regained by repeating the original treatment.

Tests for Dezincification Resistance

BS2872 and BS2874 specify a test for resistance of samples of CZ132 brass to dezincification. This involves exposure to a 1% solution of cupric chloride at 75oC for 24 hours followed by examination of sections to establish the maximum depth of any dezincification that has occurred. The sample passes the test if the maximum depth of dezincification in a forging or in the transverse direction of extruded material does not exceed 100m m. A maximum depth of 200m m is permitted in the longitudinal direction of extruded material. The European Standard version of this test is referenced in BS EN ISO 6509 and the maximum permitted depths of dezincification are defined in product standards.

This test and these criteria for acceptance are also applied by the Water Fittings Approvals Board to fittings made from brasses other than CZ132 which the manufacturers claim to be resistant to dezincification. Water fittings accepted by the approvals boards are listed in the board’s publication "Water Fittings". Those described therein as being of dezincification-resistant brass have been subjected to the cupric chloride test specified for CZ132 and have performed satisfactorily. They are identified by the mark ‘CR’ embossed or engraved on the side of the fitting.

It should be noted that there are some proprietary brasses that are described as "dezincification-resistant" by the manufacturers, and would be accepted as such in Scandinavia where a maximum depth of dezincification of 400m m in the cupric chloride test is permitted, but which cannot meet the 100m m maximum depth of attack which would make them acceptable as dezincification-resistant fittings in UK. Such fittings are not listed as dezincification-resistant by the approvals board and do not carry the ‘CR’ mark.

Historial background to the development of DZR brass

Two types of brass are in common use. The higher copper brasses generally contain over 63% copper and have a single-phase (alpha) structure. These are used particularly for their good cold forming properties as in deep drawing or in tube drawing. For optimum hot working properties, required for manufacture of water fittings by hot stamping, brasses of a lower copper content with a duplex (alpha-beta) structure are used.

Dezincification was first recognised as a serious problem in the alpha brass used for ships condenser tubes but alloying additions were developed which made the material immune. The same additions do not succeed with the duplex brasses because of the presence of beta phase as well as the alpha.

Dezincification first became a recognised problem with duplex brass water fittings in the late 1950’s, when certain water authorities banned the use of duplex brass fittings, after experiencing rapid blockage of hot water fittings as a result of dezincification. Research carried out by the British Non-Ferrous Metals Research Association (BNFMRA, later the BNF Metals Technology Centre) in collaboration with the Copper Development Association and the British Waterworks Association (now the National Water Council) established the relationship between the composition of supply waters and their liability to produce dezincification. The number of areas affected was not large and the problem was overcome by manufacturers developing ranges of fittings in copper or gunmetal which are immune to dezincification and could be specified for use in the areas concerned.

Later developments in the water supply industry, involving new large-scale schemes for water abstraction and treatment and facilities for interchange of water between different supply areas, revived concern about the risk of dezincification in water fittings. In 1969 the brass industry, together with BNF, set up a further programme of research aimed at developing a brass suitable for manufacture of water fittings by hot stamping but resistant to dezincification. Over the next five years this research established the range of alloying additions and the heat treatment that would provide a brass which, at the hot stamping temperature, would contain sufficient beta phase to forge satisfactorily but could by subsequent heat treatment, be converted to an all-alpha structure protected against dezincification. The laboratory work was followed by practical evaluation of the material in a wide range of waters and is described in a paper by J E Bowers and colleagues (). Their work culminated in 1980 in the publication of amendments to BS 2872 and 2874 defining the composition, mechanical properties, heat treatment and dezincification testing criteria for forgings and extruded bar in CZ132.

The results of standard tests of the acceptability of these fittings show them to be completely safe for handling potable water.

Many manufacturers now make fittings of dezincification-resistant brass CZ132 and most popular types and sizes of fittings are available from stock. High standards of quality control are required to ensure that the composition and heat treatment are correct and it has been agreed between the National Water Council (NWC), National Brass Founders Association (NBA) and the Copper Tube and Fittings Manufacturers Association (CTFMA) that fittings in dezincification resistant brass made to these standards should carry the ‘CR’ identification mark.

Although CZ132 was developed primarily for resistance to meringue dezincification in domestic plumbing systems its use is not restricted to fresh water service. Following a one-year test of a submerged sea water filter, in which suspension lugs machined from CZ132 bar showed no dezincification, while a naval brass plate containing less than 10% beta was dezincified to a depth of 150m m (), CZ132 has been accepted by Lloyd’s Register of Shipping, Yacht and Small Craft Department for through-hull fittings in sea water service.

Central heating systems

Water in these closed-circuit systems is de-aerated during heating. This suppresses dezincification, even if the water used to fill the system initially is one known to cause dezincification in aerated plumbing systems. Consequently radiator valves, pipe fittings etc. for central heating systems do not have to be of dezincification-resistant brass.

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