Key Coating Properties and Characteristics

Described below are the key properties and characteristics of the major coatings that are applied by continuous strip electrodeposition. These include zinc-base coatings, which are applied primarily on automotive sheet steel, and electrolytic coatings, which are applied on the tin mill products used in the container industry.

Pure Zinc (Electrogalvanized) Coatings. Electrogalvanized sheet has a pure zinc coating. It is currently the most widely used coated sheet in the United States for exposed body panels of automobiles because of its generally uniform coating thickness and excellent surface characteristics. Coating thicknesses range from 4 to 14 pm (30 to 100 g/m2) per side, although the most common coating masses are 8 and 10 pm (60 and 70 g/m2). For nonautomotive applications such as doors, furniture, and appliances, a thickness of as low as 1.5 pm (10 g/m2) can be used.

Typically, the electrodeposited zinc coating microstructure (Fig. 2) is a single-phase pure zinc, featureless, in unetched cross section (Ref 11). On the other hand, the coating surface is characterized by crystallographic facets of the hexagonal zinc crystals. Surface morphology, coating texture, and grain size may vary, depending on the chemistry of the plating solution employed as well as the specific deposition conditions of current density, temperature, and level of trace contaminants or additives (Ref 12). The zinc coating does not significantly alter the surface roughness of the steel sheet, and thus it does not affect appearance after painting. In addition, zinc galvanically protects the steel if a corrosive environment penetrates the paint layer.

Fig. 2 Surface morphology and section. Source: Ref 11

Zinc-nickel alloy coatings generally contain 10 to 14 wt% Ni. Coating thicknesses are typically 3 to 6 pm (20 to 40 g/m2). Zn-Ni coated sheet steel is used for both exposed and unexposed automobile body panels. Although Zn-Ni electrodeposited strip has been available in the United States since the 1950s in narrow widths for automotive trim and nonautomotive applications, its production and use on wide sheet for body panels started in Japan in 1983-1984 (Ref 13). Currently, it is the most widely used electroplated automotive sheet steel in Japan, second only to Zn-Fe hot-dip coated sheet for auto bodies.

Zn-Ni coated automotive sheet is coated either with Zn-Ni only (Ref 14) or with Zn-Ni plus a thin organic coating. The latter product was initially developed in Japan (Ref 15) and is now available in the United States and Europe. The organic coating system normally consists of a proprietary chromate chemical pretreatment of the order of 20 to 100 mg/m2 of chromium and a thin (about 1 pm) organic topcoat (epoxy or urethane-based) containing silicates. The organic coating was originally applied on one side of the sheet, protecting unexposed surfaces against inside-out perforation corrosion.

However, a two-side organic/Zn-Ni composite system, with chromate and organic layers applied on both sides of the sheet, is also now available and is used by at least one automaker.

In cross section, Zn-Ni coating microstructure is typically fine-grained, single-phase (gamma), Zn-Ni intermetallic. The surface is generally nodular and lacks the sharp crystallographic facets of electroplated zinc (Fig. 3). Reactivity of the Zn-Ni coating in wet salt environments, typically simulated by the salt spray test, is about three times lower than that of pure zinc. Its superior performance in salt spray and fog tests prompted its use as a thinner and more cost-effective alternative to pure zinc.

Fig. 3 Surface morphology and microstructure of zinc-nickel alloy coated sheet. Scanning electron microscope section. Source: Ref 11

Zinc-Iron Alloy Coatings. The electroplated Zn-Fe coating was developed mainly for automotive steel sheet for exposed panels, where the characteristics of the Zn-Fe hot-dip coating are desired but the uniformity and surface appearance of an electroplated coating are also required. However, recent advances in hot-dip coating technology have improved the quality of the hot-dip Zn-Fe coating or so-called Galvanneal coating. This has contributed to a decline in the production and use of the electroplated Zn-Fe coating, along with the fact that the Zn-Fe process is very difficult to operate at high speeds and volumes when insoluble anodes and sulfate-based electrolyte are used. The electrolyte, which should contain mainly ferrous iron, is easily oxidized to ferric iron, by both air and electrochemical oxidation at insoluble anodes. Therefore, such electroplating facilities must have added equipment or peripheral chemical processing plants for reducing or removing ferric iron and maintaining electrolyte stability. However, there are also chloride-based Zn-Fe processes used in the United States, Japan, and Korea that use soluble anodes to produce a uniform alloy composition over a fairly broad range of plating parameters, without the operational problems of the sulfate-based Zn-Fe electrolyte and insoluble anode process.

The electrodeposited Zn-Fe coating (Ref 16) normally contains 10 to 20 wt% Fe and is used in thicknesses up to 7 pm (50 g/m2) per side. The coating microstructure for Zn-Fe may show fine layers corresponding to the number of plating cells in the line (Fig. 4). The surface morphology appears as nodular but becomes smoother with increasing iron content. At certain automakers, when this type of coating is used for exposed automotive panels, the Zn-Fe sheet is further electroplated with a second but much thinner top layer of iron-rich Zn-Fe coating, 3 to 5 g/m2 of Zn-Fe containing at least 80 wt% Fe (Ref 17). The iron-rich top layer was developed to improve the paintability and formability of the primary zinc-rich alloy coating.

Fig. 4 Surface morphology and microstructure of zinc-iron alloy coated sheet. Scanning electron microscope section. Source: Ref 11

Electrolytic Tin Coatings. The continuous electrodeposition of tin on wide strip was introduced in 1940 (Ref 6), and wartime conditions stimulated its development. The electrodeposition process offers three main advantages over the hot-dip process: (a) higher-speed continuous strip processing; (b) better control and uniformity of coating thickness, allowing the use of thinner tin coatings; and (c) ability to plate different coating thicknesses on each surface of the strip, producing differential tinplate that can more cost-effectively meet the different corrosion resistance requirements of the inside and outside of containers.

Tin coatings are of the order of 0.4 pm thick, although they are usually expressed in terms of coating mass. Present values range from about 0.5 to 11 g/m2 on each surface. In the United States, tin coatings have numbers (Table 7) that designate the total weight of tin (i.e., the weight of the tin on the two sides per base box, a measure of surface area equal to 31,360 in.2, originally defined as 112 sheets, 14 by 20 in.). Presently there is a tendency, for economical and technological reasons, to apply lower-tin coatings, most commonly No. 20 or 25 (2.2 or 2.8 g/m2).

Table 7 Electrolytic tin coating weight and mass designations

Designation No.

Nominal tin coating weight each surface, lb/base box(a)

Minimum average coating weight each surface test value, lb/base box(a)(b)

Coating weights per ASTM A 624

10

0.05/0.05

0.04/0.04

20

0.10/0.10

0.08/0.08

25

0.125/0.125

0.11/0.11

35

0.175/0.175

0.16/0.16

50

0.25/0.25

0.23/0.23

75

0.375/0.375

0.35/0.35

100

0.50/0.50

0.45/0.45

D50/25(c)

0.25/0.125

0.23/0.11

D75/25(c)

0.375/0.125

0.35/0.11

D100/25(c)

0.50/0.125

0.45/0.11

D100/50(c)

0.50/0.25

0.45/0.23

D135/25(c)

0.675/0.125

0.62/0.11

Nominal tin coating mass each surface, g/m2

Minimum average coating mass each surface test value, g/m2(d)

Coating masses per ASTM A 624M

1.1/1.1

0.9/0.9

2.2/2.2

1.8/1.8

2.8/2.8

2.5/2.5

3.9/3.9

3.6/3.6

5.6/5.6

5.2/5.2

8.4/8.4

7.8/7.8

11.2/11.2

10.1/10.1

D5.6/2.8(c)

5.2/2.5

D8.4/2.8(c)

7.8/2.5

D11.2/2.8(c)

10.1/2.5

D11.2/5.6(c)

10.1/5.2

D15.2/2.8(c)

14.0/2.5

Note: Listed above are the commonly produced coating weights and masses. Upon agreement between the producer and the purchaser, other combinations of coatings may be specified and the appropriate minimum average test values will apply.

Source: Ref 5

(a) Base box is a measure of surface area equal to 31,360 in.2.

(b) The minimum value shall be not less than 80% of the minimum average tin coating weight.

(c) The letter D on differentially coated tin plate indicates the coated surface to be marked. For example, the examples indicate that the heavy-coated side is marked.

(d) The minimum spot value shall be not less than 80% of the minimum average tin coating mass.

Electrolytic tinplate is produced in two primary finishes. The first type is the so-called matte-finish tinplate, which is used simply with the as-deposited tin coating with its nonreflective microcrystalline surface morphology (Fig. 5). Matte-finish tinplate is used primarily for drawn and ironed two-piece containers for beverages and, more recently, for foods, with the body of the container being a single piece and the end being the second piece. The second and more common type is bright tinplate, which results when strip is heated above the melting point of tin and is quenched with water to form a bright, reflective surface. The high reflectivity and smoothness of this type of tinplate facilitates lithography, and it provides a good appearance for the end product, which is usually three-piece containers. Several variations of decorative effects and surface finishes can be obtained by varying the surface of the work rolls in the final temper-rolling or cold-rolling pass.

Fig. 5 Surface morphology and microstructure of tinplate. Scanning electron microscope section

Chromium Coatings. The chromium coating that is most widely applied to steel strip by continuous electrodeposition is an extremely thin coating consisting of layers of both metallic chromium and hydrated trivalent chromium oxides. The coating is specified by the coating weight range of the metallic chromium, 30 to 140 mg/m2 Cr, and by the amount of chromium present as the oxide, 8 to 27 mg/m2 (Ref 5). Electrolytic chromium-coated steel is further coated with a lubricating film and is used principally for can ends, closures as well as drawn bodies. However, it requires the application of an organic coating or plastic film to minimize abrasion, corrosion, and external rusting.

0 0

Post a comment