Applications

In the presence of moisture, zinc becomes a sacrificial protecting agent when in contact with iron and other metals that are below zinc in the galvanic series. Attack is most severe when the electrolyte has high electrical conductivity (as in marine atmospheres) and when the area ratio of zinc to the other metals is small.

Plate Thickness. The life of a zinc coating in the atmosphere is nearly proportional to the coating thickness. Its rate of corrosion is highest in industrial areas, intermediate in marine environments, and lowest in rural locations. Corrosion is greatly increased by frequent dew and fog, particularly if the exposure is such that evaporation is slow.

Table 8 gives the estimated life of different thicknesses of unprotected zinc coatings on steel in different outdoor atmospheres. The majority of zinc-plated parts are coated with a thickness of 7.5 to 12.5 ^m (0.3 to 0.5 mil). Typical applications employing thicknesses less than or greater than usual are given in Table 9.

Table 8 Estimated average service life of un

Condition

Coating thickness

Service, yr

mil

Rural

5

0.2

3

13

0.5

7

25

1.0

14

38

1.5

20

50

2.0

30

Temperate marine

5

0.2

1

13

0.5

3

25

1.0

7

38

1.5

10

50

2.0

13

Industrial marine

5

0.2

1

13

0.5

2

25

1.0

4

38

1.5

7

50

2.0

9

Severe industrial

5

0.2

0.5

13

0.5

1

25

1.0

3

38

1.5

4

50

2.0

6

Table 9 Applications of zinc plating at thicknesses below or above 7 to 13 pm (0.3 to 0.5 mil)

Application

Plate thickness

^m

mil

Less than 7 ^m (0.3 mil) of zinc

Automobile ashtrays1-3-1

5-7

0.2-0.3

Birdcages'®

5

0.2

Electrical outlet boxes'®

4-13

0.15-0.5

Tacks

5

0.2

Tubular rivets'®

5

0.2

More than 13 ^m (0.5 mil) of zinc

Conduit tubing1®

30

1.2

(a) Chromated after plating.

(b) Chromated after plating; some parts dyed and lacquered.

(c) Bright chromated after plating.

(d) Chromated, clear or colored, after plating.

(e) Dipped in 0.5% HNO3 or chromated after plating

Supplementary Coatings. Because corrosion is rapid in industrial and marine locations, zinc-plated parts that must endure for many years are usually protected by supplementary coatings. Steel with 5 pm (0.2 mil) of electroplated zinc is often painted to obtain a coating system for general outdoor service; a phosphate or chromate post-plating treatment ensures suitable adherence of paint to zinc.

In uncontaminated indoor atmospheres, zinc corrodes very little. A 5 pm (0.2 mil) coating has been known to protect steel framework on indoor cabinets for more than 20 years. Atmospheric contaminants accelerate corrosion of zinc if condensation occurs on cooler parts of structural members inside buildings. In 10 years or less, 12.5 pm (0.5 mil) of zinc may be dissipated. Zinc-plated steel in such locations is usually given a protective coating of paint.

A satisfactory coating for parts such as those on the inside of an office machine must afford protection in storage, assembly, and service. The cost is also important. Gears, cams, and other parts of the working mechanism can be plated with 3.8 to 6.3 ^m (0.15 to 0.25 mil) of zinc to meet these requirements.

Chromate conversion coatings, colored or clear, are almost universally applied to zinc-plated parts for both indoor and outdoor use to retard corrosion from intermittent condensation, such as may occur in unheated warehouses. Chromate films minimize staining from fingerprints and provide a more permanent surface appearance than bare zinc.

Limitations. Zinc-plated steel is not used for equipment that is continually immersed in aqueous solutions. It must not be used in contact with foods and beverages because of dangerous health effects. Although zinc may be used in contact with gases such as carbon dioxide and sulfur dioxide at normal temperatures if moisture is absent, it has poor resistance to most common liquid chemicals and to chemicals of the petroleum and pharmaceutical industries.

Fasteners. Steel fasteners, such as screws, nuts, bolts, and washers, are often electroplated for corrosion resistance and appearance. If protection against atmospheric corrosion is the sole objective, zinc is the most economical coating metal. Coatings of 5 to 7.5 ^m (0.2 to 0.3 mil) give protection for 20 years or more for indoor applications in the absence of frequent condensation of moisture. Chromate coatings are used to retard corrosion from condensates, provide a more permanent surface appearance, and prevent staining from fingerprints. For indoor use in industrial areas and in locations where condensation is prevalent, as in unheated buildings, corrosion may be rapid, and the zinc surface should be phosphated and then painted to extend its service beyond the few years that would be obtained by the unpainted coating. Unprotected zinc-plated screws should not be used to fasten bare parts if the service is to include marine exposure.

The dimensional tolerance of most threaded articles, such as nuts, bolts, screws, and similar fasteners with complementary threads, does not permit the application of coatings much thicker than 7.5 ^m (0.3 mil). The limitation of coating thickness on threaded fasteners imposed by dimensional tolerance, including class or fit, should be considered whenever practicable, to prevent the application of thicker coatings than are generally permissible. If heavier coatings are required for satisfactory corrosion resistance, allowance must be made in the manufacture of the threaded fasteners for the tolerance necessary for plate buildup. If this is not practicable, phosphating before assembly and painting after assembly will increase service life. The approximate durability of 5 ^m (0.2 mil) untreated coatings is given in Table 8.

Appearance. The appearance of electrodeposited zinc can be varied over a wide range, depending on bath composition, current density, the use of brighteners, and postplating treatments. The appearance of electroplated zinc is bright and silvery, and the deposit from the acid chloride baths is often initially indistinguishable from bright nickel chrome when plated.

Currently, nearly all zinc plating is followed by some type of chromate dip. These preserve the appearance of the part and vastly increase the bright shelf life of the surface. The cost of chromating is so minimal that its use has become practically universal. Presently, bright zinc deposits are used for a wide variety of low-cost consumer goods such as children's toys, bird cages, bicycles, and tools. Refrigerator shelves are commonly bright zinc plated, chromated, and lacquered. Without lacquer protection, even chromated bright zinc will tarnish and discolor quite rapidly when handled, and unlacquered bright zinc plate is not a good substitute for nickel chrome when a longlasting bright finish is desired. However, the vast majority of zinc plate is deposited primarily to impart corrosion resistance; brightness is not the primary factor for these applications.

Additional information about applications of electroplated zinc is provided in the article "Surface Engineering of Carbon and Alloy Steels" in this Volume.

Indium Plating

Allen W. Grobin, Jr., Grobin Associates, Inc.

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