Zinc Diecasting Alloys

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Corrosion Properties

Corrosion Resistance

Zinc has excellent resistance to corrosion in most natural environments and this has given rise to its extensive use for corrosion protection. In most circumstances the behaviour of die cast zinc alloys are similar to that of unalloyed zinc and if in any application zinc or galvanized steel is known to be satisfactory, zinc die castings will usually also behave well; the exception is in severe marine environments (see below). Table 1 compares the rates of corrosion of high purity zinc and of zinc diecastings in a number of different environments. Diecast zinc alloy like other forms of zinc generally corrodes at an even, slow rate; castings that meet the requirements of EN 12844 (and similar specifications in other parts of the world) are not subject to intercrystalline corrosion. This form of corrosion, which can lead to rapid failure, can only occur when harmful impurities, particularly tin and lead, exceed the specified levels; in most countries reputable zinc die casters belong to schemes which guarantee the maintenance of strict analytical control on the alloy so that impurities never reach such levels.

The literature on the corrosion resistance of zinc in all its forms is very large indeed. It has been reviewed in a monograph Zinc: ‘Its Corrosion Resistance – reference 11‘ to which reference should be made for more detailed information than is contained here.

Corrosion in the Atmosphere

When exposed out-of-doors unprotected zinc alloy diecastings soon lose their bright metallic appearance and acquire a compact and closely adherent coating of corrosion products, which reduces the rate of subsequent attack.

The table below shows the range of rates of corrosion that are likely to be encountered with zinc die castings; the rate of corrosion is unlikely to exceed 12 microns/yr and may be as low as one-tenth of this. The reduction in SO2 levels in the atmosphere in Europe over recent years has brought about a marked reduction in the rate of corrosion of zinc surfaces in urban and many industrial environments. The corrosion rates shown in the table below for industrial environments may well be significantly higher than that which can now be expected in practice.

Where the higher rates of corrosion encountered under severe industrial and marine conditions must be prevented, a protective finish coating will be required.

Corrosion Rates of Zinc and Zinc Alloys in Various Atmospheres

Material Location Type of atmosphere Corrosion
micron/yr
High purity zinc Khartoum. Sudan dry tropical 0.25
High purity zinc Abisto. North Sweden sub polar 0.25
High purity zinc Cardington.Bedfordshire.UK rural 3.00
High purity zinc State College. Pennsylvania. USA rural 1.00
High purity zinc Wakefield.Yorkshire.UK industrial 6.00
High purity zinc New York. USA industrial 7.00
Diecast zinc alloy New York. USA industrial 6.00
High purity zinc Euston, London.UK industrial 10.00
High purity zinc Key West, Florida. USA marine 3.00
Diecast zinc alloy Key West. Florida. USA marine 10.00
High purity zinc Montauk Point. Long Island, USA marine 3.00
Diecast zinc alloy Montauk Point. Long Island, USA marine 10.00

Reference 11

Under any conditions -indoors or outdoors -where condensation can form on the surface of the zinc and remain there for extended periods, a conspicuous form of white corrosion product resembling the ‘white rust’ which forms on galvanized surfaces in similar conditions will be produced on die castings. This form of corrosion does not normally cause a very rapid rate of metal loss but wherever it needs to be prevented for reasons of appearance a conversion coating can provide the necessary protection. This form of protection is recommended when die castings are to be transported through or used in the tropics in unprotected situations or in exterior coastal environments.

Since the effect of corrosion on mechanical properties is often important, the changes in mechanical properties during 20 years exposure at various sites is summarised in the table below; this provides valuable evidence of the durability of zinc die castings.

Combined Effect Of Exposure and Ageing on the Mechanical Properties of Pressure Diecast Zinc Alloy (ASTM AG 4OA, equivalent to ZP3).

New York.
NY
(outdoor)
Industrial/ Marine
Altoona,
Pennsylvania
(outdoor) Urban
State College.
Pennsylvania
(outdoor)Urban/Rural
Coco Solo Islands.
Canal Zone
(indoor)
Tropical
Tucson.
Arizona
(indoor)Desert
Tensile strength MPa
Original 253 253 252 253 253
Five years 236 230 242 233 249
Ten years 213 223 232 234 235
Twenty years 207 209 212 223 232
Elongation %
on 50.8mm
Original 15 15 15 15 15
Five years 9 18 16 20 16
Ten years 16 9 11 23 18
Twenty years 6 8 18 21 20
Hardness
(Rockwell E)
Original 83 83 83 83 83
Five years 78 78 82 68 77
Ten years 81 70 75 76 79
Twenty years 68 75 67 71 77
Impact strength (Charpy) J
Original 53 53 53 53 53
Five years 57 41 52 44 59
Ten years 44 60 39 59 54
Twenty years 14 19 52 50 57
Dimensional change %
Five years -0.01 -0.01 -0.01 -0.01 -0.01
Ten years -0.02 -0.01 0.00 0.00 -0.01
Twenty years -0.02 -0.02 0.00 0.00 -0.01

Reference 13

The above figures are presented graphically here below

ima01

ima02

ima03

ima04

Diecastings with Applied Finishes

Where castings are to be used over extended periods in aggressive environments or where the matt grey appearance of weathered zinc is not acceptable, zinc diecastings are often finished by a variety of different processes.

Zinc die castings are often powder coated or painted and in these instances it is worthwhile to remember that organic coatings on zinc last longer than would a similar coating on iron or steel. When in the normal course of events the paint coating cracks or pores develop, probably as a result of weathering, complete failure follows quickly when the coating is on iron or steel, as corrosion spreads along the interface between the steel and the coating and lifts off substantial areas rather quickly. On zinc however corrosion does not spread along the interface and the effective life of the paint coating is therefore extended.

Zinc alloy diecastings are also often “chromium” plated. Where this finish is applied, at least in part to provide corrosion protection, it is important that sufficient thicknesses of copper and nickel undercoats are applied for the anticipated service conditions (see EN 12540:2000).

Corrosion in Gases Other than Air

Oxygen, nitrogen, carbon dioxide and fuel and inert gases do not corrode zinc, but if moisture is present ‘wet storage stain’ may be formed. Castings should be passivated where it is necessary to prevent this. Zinc die castings are used for some natural gas supply parts (e.g. gas regulators) and have been in use for components of bottled gas equipment for many years.

Contact with Fresh Water

Die cast zinc water fittings are used with satisfaction in some parts of the world but the rate of corrosion of zinc in fresh supply waters varies greatly according to the hardness, chemical composition and acidity of the water. The rates vary substantially but the range is generally from 2.5 to 100 microns per year. Hard waters generally cause less corrosion because the calcium carbonate scale that becomes deposited on the surface reduces the rate of attack. The source of the water can also have an important effect particularly on the rates of corrosion in hot water, as waters from surface sources contain minute amounts of organic compounds which help the scale to be deposited in a firm adherent form, rather like an egg-shell, which has maximum protective effect. Waters from deep wells generally deposit the scale in an irregular and less adherent form that gives much less protection to the zinc. The pH of most potable waters is in the range 5.0 to 8.5; corrosion of zinc is generally lowest in the range 6.5 to 12. Where it is contemplated to use zinc die castings in continuous contact with supply waters detailed investigation should be made as to whether the zinc can be expected to perform well without some sort of protective coating. Factors such as free carbon dioxide are also important and the most rapid rates of attack are experienced with waters with high carbon dioxide content and a very low concentration of dissolved solids

Contact with Seawater

Experience with the use of zinc die castings in seawater is not extensive but experience with other forms of zinc suggest that for conditions of total immersion a corrosion rate of 12 to 25 microns per year can be expected. Rates of corrosion will increase quite considerably when immersion is intermittent, as in the tidal zone, and whilst the corrosion rate for total immersion may be tolerated for some applications adequate protection should be provided for castings subject to intermittent immersion. Corrosion will be accelerated by contact with other metals.

Contact with other Aqueous Solutions

As a general rule, zinc diecastings should only be used in contact with solutions of pH between 6.5 and 12, as the rate of corrosion increases rapidly outside this range. However, a number of other factors such as agitation, temperature and polarization can be important and may substantially change the rate of attack from what would otherwise be expected. Detailed results of a number of different solutions have been collected together in the manual ‘Zinc: Its Corrosion Resistance’ otherwise tests should be made on the suitability of die castings for use in contact with any aqueous solution detergents and soaps. Most detergents are mildly alkaline and therefore have only a slow rate of attack, which is acceptable for most cases. Concentrated or very hot detergents solutions may attack unprotected zinc castings more rapidly.

Contact with Organic Materials

In general organic chemicals have little effect on zinc unless water or an acid component are either present naturally or formed as part of the usual products of atmospheric breakdown. For example, trichlorethylene does not react with zinc diecastings and neither does anhydrous alcohol but increased corrosion may be expected when water is present in either case. Refined oils do not attack zinc unless they contain appreciable amounts of sulphur, water or acid compounds. For example, petrol containing water can result in corrosion products which might block fuel injector jets. Chromate treatment has been found to prevent this, but zinc fuel system parts rarely need to be chromated nowadays. Diesel oil containing sulphur can also produce compounds that block fuel jets. Lubricants based on animal products should be used with caution because they contain acids formed by oxidation, but purely mineral lubricants are generally satisfactory. The effects of a large number of substances are tabulated in the monograph ‘Zinc :Its Corrosion Resistance‘.

Contact with Food and Drink

As-cast surfaces of zinc alloy should not be used in contact with acid foodstuffs unless they may be expected to remain dry. Otherwise the zinc must be adequately protected by copper-nickel-chromium plating or another satisfactory impervious coating. The slight acidity present in many foodstuffs can attack the zinc and might give the food an unpleasant metallic taste. For the same reason zinc die castings used in any equipment to hold or dispense drinks should also be plated or otherwise protected. Although zinc is an essential micronutrient for good health, consumption of food very highly contaminated with zinc may cause vomiting. It is not dangerous and this rarely arises because of the taste.

Bi-Metallic Contact Corrosion

When two different metals are in contact with a corrosive environment they form an electrolytic cell and a current flows from the baser (anodic) metal to the nobler (cathodic) metal. As a result the corrosion rate of the nobler metal tends to decrease and that of the baser metal to increase compared with the rates for the same metals not in contact but subject to the same corrosive conditions. The driving force for this electrolytic reaction is the potential difference between the metals. Although this can be measured in a laboratory, (see the electrochemical series table below) it is not a reliable guide to the severity of the increase in corrosion that may result from bi-metallic contact. This phenomenon is the basis of a method used to protect steel in some circumstances (cathodic protection). Specially shaped pieces of zinc are placed in electrical contact with the steel to be protected (frequently steel structures in the sea or buried pipelines) and the zinc is steadily consumed whilst the steel is protected from corrosion. Bi-metallic corrosion of zinc diecastings rarely proves to be a problem. Many millions of castings are in service, which have inserts of brass, bronze and other nobler metals, and these have not shown any appreciable increase in the rate of corrosion of the zinc. In general therefore it is reasonable to assume that under mild or moderately severe atmospheric corrosion conditions some increase of corrosion can be expected but this is unlikely in many instances to affect the serviceability of the components. Under immersed conditions contact with copper and copper alloys should be avoided.

Reference 13

Additional Corrosion of Zinc and Zinc Base Alloys
Resulting from Contact with Other Metals or Carbon

Metal. in contact
Environment
Atmospheric
Immersed
Rural
Industr./urban
Marine
Fresh water
Sea water
Aluminum and aluminum alloys
0
0 to 1
0 to 1
1
1 to 2
Aluminum bronzes and silicon bronzes
0 to 1
1
1 to 2
1 to 2
2 to 3
Brasses including high tensile (HT),brass (manganese bronze).
0 to 1
1
0 to 2
0 to 2
2 to 3
Cadmium
0
0
0
0
0
Carbon
0 to 1
1
1 to 2
1 to 2
2 to 3
Cast irons
0 to 1
1
1 to 2
1 to 2
2 to 3
Cast iron (austenitic)
0 to 1
1
1 to 2
1 to 2
1 to 3
Chromium
0 to 1
1 to 2
1 to 2
1 to 2
2 to 3
Copper
0 to 1
1 to 2
1 to 2
1 to 2
2 to 3
Cupro-nickels
0 to 1
0 to 1
1 to 2
1 to 2
2 to 3
Gold
0 to 1
1 to 2
1 to 2
1 to 2
2 to 3
Gunmetals, phosphor bronzes and tin
Bronzes
0 to 1
1
1 to 2
1 to 2
2 to 3
Lead
0
0 to 1
1 to 2
0 to 1
0 to 2
Magnesium and magnesium alloys
0
0
0
0
0
Nickel
0 to 1
1
1 to 2
1 to 2
2 to 3
Nickel copper alloys
0 to 1
1
1 to 2
1 to 2
2 to 3
Nickel-chromium-iron alloys
0 to 1
1
1 to 2
1 to 2
1 to 3
Nickel-chromium-molybdenum alloys
0 to 1
1
1 to 2
1 to 2
1 to 3
Nickel silvers
0 to 1
1
1 to 2
1 to 2
1 to 3
Platinum
0 to 1
1 to 2
1 to 2
1 to 2
2 to 3
Rhodium
0 to 1
1 to 2
1 to 2
1 to 2
2 to 3
Silver
0 to 1
1 to 2
1 to 2
1 to 2
2 to 3
Solders hard
0 to 1
1
1 to 2
1 to 2
2 to 3
Solders soft
0
0
0
0
0
Stainless steel (austenitic and other grades containing approximately 18%chromium)
0 to 1
0 to 1
0 to 1
0 to 2
1 to 2
Stainless steel (martensitic grades
containing approximately 13% chromium)
0 to 1
0 to 1
0 to 1
0 to 2
1 to 2
Steels (carbon and low alloy)
0 to 1
1
1 to 2
1 to 2
1 to 2
Tin
0
0 to 1
1
1
1 to 2
Titanium and titanium alloys
0 to 1
1
1 to 2
0 to 2
1 to 3
Zinc and zinc base alloys
0
0
0
0
0

Key
0. Zinc and zinc base alloys will suffer either no additional corrosion, or at the most only very slight additional corrosion, usually tolerable in service.
1. Zinc and zinc base alloys will suffer slight or moderate additional corrosion that may be tolerable in some circumstances.
2. Zinc and zinc base alloys may suffer fairly severe additional corrosion and protective measures will usually be necessary.
3. Zinc and zinc base alloys may suffer severe additional corrosion and the contact should be avoided.

General notes: Ratings in brackets are based on very limited evidence and hence are less certain than other values shown. The table is in terms of additional corrosion and the symbol 0 should not be taken to imply that the metals in contact need no protection under all conditions of exposure.

Reference 14

Electrochemical Potential/Series

Element
Reaction
Electrode
Potential
Volts
Gold Au+ + e = Au 1.692
Gold Au3+ + 3 e = Au 1.498
Platinum Pt2+ + 2 e  = Pt 1.18
Palladium Pd2+ + 2 e = Pd 0.951
Silver Ag+ + e = Ag 0.7996
Copper Cu+ + e = Cu 0.521
Copper Cu2+ + 2 e = Cu 0.3419
Iron Fe3+ + 3 e = Fe -0.037
Lead Pb2+ + 2 e = Pb -0.1262
Tin Sn2+ + 2 e  = Sn -0.1375
Nickel Ni2+ + 2 e = Ni -0.257
Cobalt Co2+ + 2 e  = Co -0.28
Cadmium Cd2+ + 2 e = Cd -0.403
Iron Fe2+ + 2 e = Fe -0.447
Chromium Cr3+ + 3 e = Cr -0.744
Zinc Zn2+ + 2 e = Zn -0.7618
Chromium Cr2+ + 2 e = Cr -0.913
Manganese Mn2+ + 2 e = Mn -1.185
Titanium Ti3+ + 3 e = Ti -1.37
Titanium Ti2+ + 2 e = Ti -1.63
Aluminium Al3+ + 3 e = Al -1.662
Magnesium Mg2+ + 2 e = Mg -2.372
Magnesium Mg+ + e = Mg -2.7
Sodium Na+ + e = Na -2.71
Calcium Ca2+ + 2 e = Ca -2.868
Potassium K+ + e = K -2.931
Lithium Li3+ + e = Li -3.0401
Calcium Ca+ + e = Ca -3.8

Table Of Potential Differences Between Zinc and a Number
of Other Common Metals and Alloys in 2% Salt Solution 

Material
Composition
Potential
millivolts
Platinum
1400
Gold
1270
Stainless steel, passive 18%Cr 8%Ni
1150
Chromium, passive
1150
Silver
1050
Mercury
1050
Nickel
970
Arcap Cu55% Zn23% Ni 22%
950
Copper
830
Aluminium Bronze Cu90% Al10%
800
Brass Cu Zn39%
750
Bronze Cu88% Sn12%
630
Tin
600
Lead
560
Mild steel 0.08 – 0.12% C
400
Hard steel 0.8 – 1.2% C
305
Cadmium
300
Iron
295
White metal Sn 75% Zn25%
40
Zinc
0
Almasilium Al 5% Mg
-295
Duralinox Al Mg3%   Al Mg5%
-300
Aluminium 99.5%
-310
Alpax H Al Si 10% Mg
-335
Duralumin Al Cu4% Mg
-530

Reference 15