Precoated Steel Sheet(1)

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Precoated Steel Sheet(1)

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Precoated Steel Sheet(1)

Messaggioda Aldebaran » 10/05/2010, 13:41

Precoated Steel Sheet
Revised by R. W. Leonard, USS Corporation, Division of USX Corporation
Introduction
STEEL SHEET is often coated in coil form before fabrication either by the steel mills or by specialists known as coil
coaters. This prefinished or precoated sheet is ready for fabrication and use without further surface coating. Precoated
products yield lower production costs, improved product quality, shorter processing cycles, elimination of production
hazards, conservation of energy, minimized ecological problems, and production expansion without a capital expenditure
for new buildings and equipment.
Some precautions are necessary with pre-coated sheet. The product must be handled with more care to prevent scratches
and damage to the prefinished surface. Metal finishing of damaged areas is more difficult than on uncoated sheet.
Fabrication methods are more restrictive, bend radii must be more generous, and welding practices must be carefully
chosen.
The basic types of precoating include metallic, pretreated, preprimed, and pre-painted finishing. Metallic coating can be
made up to zinc, aluminum, zinc-aluminum alloys, tin, and terne metal. Pretreatment coatings are usually phosphates, and
pre-primed finishes can be applied as a variety of organic-type coatings. These can be used as a primed-only coating, or a
suitable paint topcoat can be applied. Prepainting consists of applying an organic paint system to steel sheet on a coil
coating line either at a mill or at a coil coater. This article will address each of these coating processes. Emphasis will be
placed on products that are galvanized by the hot dip process, although much of the discussion is equally applicable to
electrogalvanizing and zinc spraying.
Zinc Coatings
Galvanizing is a process for rustproofing iron and steel by the application of a metallic zinc coating. It is applicable to
products of nearly all shapes and sizes, ranging from nails, nuts, and bolts to large structural assemblies and steel sheet in
coils and cut lengths. Other applications include roofing and siding sheets for buildings, silos, grain bins, heat exchangers,
hot water tanks, pipe, culverts, conduits, air conditioner housings, outdoor furniture, and mail boxes. On all steel parts,
galvanizing provides long-lasting, economical protection against a wide variety of corrosive elements in the air, water, or
soil.
In the United States, more than 9 × 106 Mg (1 × 107 tons) of steel is produced annually by precoating. A large amount of
this total is used by the automotive industry for both unexposed and exposed panels--from frames and floor pans to doors,
fenders, and hoods (Fig. 1). Typically, 75% of the body, chassis, and power train components of one American
automobile manufacturer's 1986 models consisted of galvanized precoated sheet (Fig. 2). Table 1 indicates that a typical
1986 American car utilized nearly 160 kg (350 lb) of zinc-coated steel components in its material composition. As
indicated in Table 2, undervehicle test coupons evaluated after 2 years of exposure attest to the benefits of precoated
steels in combating corrosion (additional information is available in the article "Corrosion in the Automobile Industry" in
Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook.
Metallic zinc is applied to iron and steel by three processes: hot dip galvanizing, electrogalvanizing, and zinc spraying.
Most galvanized steel sheet is coated by the hot dip process, although there has been strong growth in electrogalvanizing
capacity during the past few years.
Corrosion Resistance. The use of zinc is unique among methods for the corrosion protection of steel. The zinc coating
serves a twofold purpose:
· It protects the steel from corrosive attack in most atmospheres, acting as a continuous barrier shield
between the steel and the atmosphere
· It acts as a galvanic protector, slowly sacrificing itself in the presence of corrosive elements by
continuing to protect the steel even when moderate-sized areas of bare metal have been exposed
This latter ability is possible because zinc is more electrochemically active than steel. This dual nature of zinc coatings is
also available with some zinc/aluminum alloy coatings, but zinc coatings clearly offer the most galvanic protection. With
most protective coatings that act only as a barrier, rapid attack commences when exposure of the base metal occurs.
The distance over which the galvanic protection of zinc is effective depends on the environment. When completely and
continuously wetted, especially by a strong electrolyte (for example, seawater), relatively large areas of exposed steel will
be protected as long as any zinc remains. In air, where the electrolyte is only superficially or discontinuously present
(such as from dew or rain), smaller areas of bare steel are protected. The order of magnitude of this throwing power is
nominally about 3.2 mm ( 1
8
in.), although this can vary significantly with the type of atmosphere. Nevertheless,
galvanized parts exposed outdoors have remained rust free for many years, and the two basic reasons are the sacrificial
protection provided by the zinc and the relatively stable zinc carbonate film that forms on the zinc surface to reduce the
overall corrosion rate of the zinc coating.
The service life of zinc-coated steel is dependent on the conditions of exposure and on the coating thickness, as illustrated
in Fig. 3. Although the coating process used to apply the zinc coating generally does not affect the service life, experience
has shown that the corrosion resistance of galvanized coatings in the field cannot be accurately predicted from accelerated
laboratory tests. Environmental factors such as atmospheric contaminants (sulfates, chlorides, and so on) and time of
wetness have a large influence on the service life of galvanized steel. In polluted areas, such as severe industrial areas, the
normally protective zinc carbonate film that forms on the surface of the zinc coating tends to be converted to soluble
sulfates that are washed away by rain, thus exposing the zinc to further attack and accelerating the corrosion.
Coating Test and Designations. The thickness (or weight), adhesion, and ductility of a zinc coating can have
important effects on its service life and effectiveness against corrosion. Practical tests for these characteristics are
described in relevant specifications issued by the American Society for Testing and Materials (ASTM). Tests for coating
thickness include microscopic measurement of the cross section, stripping tests in which the coating is removed from a
given area (ASTM A 90), electrochemical stripping from a given area (ASTM B 504), and magnetic and electromagnetic
methods of measurement (ASTM E 376, A 123, A 754, B 499, and D 1186). Adhesion can be tested and rated by bend
test methods described in ASTM A 525 and A 879. Other adhesion test methods include reverse impact and draw bend
test.
Because the service life of a zinc-coated part in a given atmosphere is directly proportional to the thickness of zinc in the
coating (Fig. 3), measurement of that amount is very important. The amount of coating is most often measured in terms of
weight rather than thickness, usually by the method described in ASTM A 90. Specimens are cut from one or three spots
in samples of the sheet, as described in ASTM A 525. These are weighed, the zinc is stripped (dissolved) in an acid
solution, and the specimens are reweighed. The weight loss is reported in ounces per square foot of sheet or grams per
square meter. When specimens from three spots are checked (triple-spot test), the value of weight loss is the average of
the three specimens.
When the weight-loss method is used, the amount of coating measured is the total amount on both sides of the sheet.
Ordinarily, the zinc coating is applied to both sides of the sheet. Therefore, a 2 oz/ft2coating has 305 g/m2 (1 oz/ft2) on
each surface. This 28 g (1 oz) is equivalent to an average thickness of 43 μm (1.7 mils). When zinc-coated sheet is
ordered, the minimum amount of coating can be specified as the weight determined by the triple-spot or single-spot test or
by coating designations corresponding to these weights.
Chromate Passivation. Several types of finishes can be applied to zinc-coated surfaces to provide additional
corrosion resistance. The simplest type of finish applicable to fresh zinc surfaces is a chromate passivation treatment. This
process is equally suitable for use on hot dip galvanized, electrogalvanized, zinc-sprayed, and zinc-plated articles.
Usually, the treatment consists of simply cleaning and dipping the articles in a chromic acid or sodium dichromate
solution at about 20 to 30 °C (68 to 86 °F), followed by rinsing in cold fresh water and drying in warm air. The adherent
chromate film may have a greenish or greenish-yellow iridescent appearance. Specification ASTM B 201 gives details of
tests for measuring the adequacy and effectiveness of the chromate film. Chromate passivation helps prevent staining
when galvanized sheet is stored under wet or humid conditions. Therefore, a thin, almost clear chromate or
chromate/phosphate passivation film is often applied to the coating on hot dip coating lines.
Painting. The selection of galvanized steel as a material for barns, buildings, roofs, sidings, appliances, and many
hardware items is based on the sacrificial protection and the barrier coating afforded the base metal by zinc coating. For
additional protection and cosmetic appearance, paint coatings are often applied to the galvanized steel. The performance
of the coatings is an important economic factor in the durability of this material.
Galvanized steel, both new and weathered, can be painted with a minimum of preparation and with excellent adherence.
On hot dip galvanized or zinc-plated steel, it is necessary to use special corrosion-inhibitive primers to prepare the surface
before the paint is applied. This is partly because these types of zinc coating are too smooth to provide a mechanical key
for the paint or lacquer and partly because the paint appears to react with the unprepared zinc surface in the presence of
moisture to weaken the initially formed bond.
Many exposure tests have shown that zinc dust-zinc oxide paints (finely powdered zinc metallic and zinc oxide pigment
in an oil or alkyd base) adhere best to galvanized steel surfaces under most conditions. Zinc dust-zinc oxide primers can
be used over new or weathered galvanized steel and can be top coated with most oil or latex house paints or alkyd
enamels. For the maintenance painting of galvanized steel, one or two coats of a zinc dust-zinc oxide paint are often used
alone. The paint can be applied by brushing, rolling, or spraying.
Zinc sheets to be painted should not be treated at the mill with a chromate treatment, although, they may be given a
phosphate treatment to improve the adherence of the paint. Zinc-coated sheet steel is often prepainted in coil form by coil
coating, is described in the section "Prepainted Sheet" in this article.
Packaging and Storage. Galvanized products in bundles, coils, or stacks of sheets must be protected from moisture,
including condensation moisture, until openly exposed to the weather. They must be properly packaged and stored.
Otherwise, wet-storage stain, a bulky white deposit that often forms on zinc surfaces stored under wet or humid
conditions, may develop.
It is important to examine packages of galvanized products for damage and to take prompt action where cuts, tears, or
other damage is evident. If the packaging is damaged or if moisture is present, the product should be dried at once and not
repiled until thoroughly dry. Erection of materials should begin as soon as possible after the package arrives at the
installation site.
If temporary storage of the galvanized product is absolutely necessary, it should be indoors. Where indoor storage is not
possible, intact waterproof bundles can be stored at the site. The package should be slanted so that any condensation will
drain out, and it should be stored sufficiently high to allow air circulation beneath and to prevent rising water from
entering. The stacks should be thoroughly covered with a waterproof canvas tarpaulin for protection from rain, snow, or
condensation. The use of airtight plastic coverings should be avoided. To deter the formation of wet-storage stain, zinccoated
sheet can be purchased with a mill-applied chromate or chromate/phospate film. Various proprietary mixtures are
available.
Hot dip galvanizing is a process in which an adherent, protective coating of zinc and iron-zinc alloys is developed on
the surfaces of iron and steel products by immersing them in a bath of molten zinc. Most zinc coated steel is processed by
hot dip galvanizing.
One method of hot dip galvanizing is the batch process, which is used for fabricated steel items such as structurals or
pipe. This method involves cleaning the steel articles, applying a flux to the surfaces, and immersing them in a molten
bath of zinc for varying time periods to develop a thick alloyed zinc coating.
The most common form of hot dip galvanizing for steel sheet is done on a continuous galvanizing line. Coiled sheet is fed
from pay-off reels through flatteners. It is then cleaned, bright annealed, and passed through the coating bath. After
leaving the coating bath, the coating thickness is controlled by an air knife or steel rolls. The sheet is then cooled and
recoiled or cut into lengths. The hot dip process normally coats both sides of the sheet. However, hot dip galvanized
sheets can be coated on one side only for special uses, such as automotive exposed panels, by the use of special coating
techniques. One-side coated sheet produced by the hot dip process is not commonly available. Continuous coating lines
have to be specially modified to make one-side coated product.
A typical hot dip galvanized coating produced by the batch process consists of a series of layers (Fig. 4). Starting from the
base steel at the bottom of the coating, each successive layer contains a higher proportion of zinc until the outer layer,
which is relatively pure zinc, is reached. There is, therefore, no real line of demarcation between the iron and zinc, but a
gradual transition through the series of iron-zinc alloys that provide a powerful bond between the base metal and the
coating. These layers are identified in Table 5. The structure of the coating (the number and extent of the alloy layers) and
its thickness depend on the composition and physical condition of the steel being treated as well as on a number of factors
within the control of the galvanizer.
The ratio of the total thickness of the alloy layers to that of the outer zinc coating is affected by varying the time of
immersion and the speed of withdrawal of the work from the molten zinc bath. The rate of cooling of the steel after
withdrawal is another factor to be considered; rapid cooling gives small spangle size.
Sheet galvanizers operating continuous strip processes usually suppress the formation of alloy layers by adding 0.1 to
0.2% Al to the bath; this increases the ductility of the coating, thus rendering the sheet more amenable to fabrication (Fig.
5). Other elements can be added to galvanizing baths to improve the characteristics and appearance of the coating. Lead
and antimony give rise to well-defined spangle effects.
During batch galvanizing, the zinc-iron alloy portion of the coating will represent 50 to 60% of the total coating thickness.
However, certain combinations of elements may result in a coating that is either completely or almost completely alloyed.
Visually, the zinc-iron alloy coating will have a gray, matte appearance because of the absence of the free-zinc layer. The
free-zinc layer imparts the typical bright finish to a galvanized coating. Because of the greater percentage of the zinc-iron
alloy present in the coating, the alloy-type coating may have a lower adherence than the regular galvanized coating.
The corrosion resistance of the zinc-iron and free zinc coating types is equal for all practical purposes. Steels containing
carbon below 0.25%, phosphorus below 0.05%, and manganese below 1.35% (either individually or in combination) will
normally develop regular galvanized coatings when conventional galvanizing techniques are used and when silicon is
0.05% or less or ranges between 0.15 and 0.3%. Fabricators and consumers should be aware that a gray matte appearance
may occur in batch galvanizing if silicon content exceeds 0.06%. This matte appearance does not reduce the long-term
atmospheric corrosion protection of the galvanized coating.
Galvanized coatings on sheet products that are intended to be painted are frequently given treatments to make the spangle
less obvious so that it does not show through the paint. A flat spangle without relief (suppressed spangle) can be obtained
by small additions of antimony to the molten bath; smaller grain size (minimized spangle) can be produced by spraying
the molten zinc with zinc dust, steam, air, or water just before it freezes. Finer grains are less visible through the paint and
have narrower and smaller fractures on bending, often permitting the paint to bridge the gap and provide increased
protection.
Galvanized steel sheet can be temper rolled to flatten surface irregularities such as dross and grain boundaries, thus
providing an extra smooth surface more suitable for painting where critical surface requirements exist. At the galvanizing
mill, galvanized steel sheet can be given a thermal treatment after coating, which converts all the free zinc to zinc-iron
alloy, thus providing a spangle-free surface that is more suitable for painting. It can be painted without pretreatment (but
not with all paints). As an added benefit, there is no spangle to show through the paint. However, the zinc-iron alloy
coating is somewhat brittle and tends to powder if severely bent in fabrication.
The typical mechanical properties of commercial quality (CQ), drawing quality (DQ), and drawing quality, special killed
(DQSK) hot dip galvanized steel sheet are listed in Table 9. Commercial quality sheet is satisfactory for applications
requiring bending and moderate drawing. Drawing quality sheet has better ductility and uniformity than commercial
quality and is excellent for ordinary drawing applications. Drawing quality, special killed sheet is superior to drawing
quality and is excellent for applications requiring severe drawing. When higher strength is required, structural quality
(SQ) sheet, also called physical quality (PQ) sheet, can be specified, although at some sacrifice in ductility (compare
Tables 7 and 8). The minimum mechanical properties of structural quality sheet are presented in Table 10. Additional
information is available in the article "Hot Dip Coatings in Corrosion, Volume 13 of ASM Handbook, formerly 9th
Edition Metals Handbook.
Electrogalvanizing. Very thin formable zinc coatings ideally suited for deep drawing or painting can be obtained on
steel products by electrogalvanizing. Zinc is electrodeposited on a variety of mill products: sheet, wire, and, in some
cases, pipe. Electrogalvanizing the sheet and wire in coil form produces a thin, uniform coating of pure zinc with
excellent adherence. The coating is smooth, readily prepared for painting by phosphatizing, and free of the characteristics
spangles of hot dip zinc coatings. Electrogalvanizing can be used where a fine surface finish is needed. The appearance of
the coating can be varied by additives and special treatments in the plating bath.
Electrodeposited zinc coatings are simpler in structure than hot dip galvanized coatings. They are composed of pure zinc,
have a homogeneous structure, and are highly adherent. These coatings are not generally as thick as those produced by
hot dip galvanizing. Electrogalvanized coating weights as high as 100 g/m2 (0.3 oz/ft2) have been applied to one or both
sides of steel sheet. The normal ranges of coating weights available are listed in ASTM Specifications A 591 and A 879.
The coating thicknesses listed in A 591 are typically used when the application does not subject the steel sheet to very
corrosive conditions or when the sheet is intended for painting. For more severe corrosion conditions, such as the need to
protect cars from road salts and entrapped moisture, heavier coatings in the ranges listed in A 879 are used. These coating
weights are applied to the steel sheets used for most body panels.
Electrodeposited zinc is considered to adhere to steel as well as any metallic coating. Because of the excellent adhesion of
electrodeposited zinc, electrogalvanized coils of steel sheet and wire have good working properties, and the coating
remains intact after severe deformation. Good adhesion depends on very close physical conformity of the coating with the
base metal. Therefore, particular care must be taken during initial cleaning. Electrodeposition also affords a means of
applying zinc coatings to finished parts that cannot be predipped. It is especially useful where a high processing
temperature could damage the part. One advantage of electrodeposition is that it can be done cold and therefore does not
change the mechanical properties of the steel.
Zincrometal is also used for outer body panels in automobiles. First introduced in 1972, Zincrometal is a coil coated
product consisting of a mixed-oxide underlayer containing metallic zinc particles and a zinc-rich organic (epoxy) topcoat.
It is weldable, formable, paintable, and compatible with commonly used adhesives. Zincrometal is primarily used in oneside
applications to protect against inside-out corrosion. The corrosion resistance of Zincrometal is not as good as that of
hot dip galvanized steels (Ref 1), and its use is declining substantially as more electrogalvanized steels and other types of
coatings are employed.
Zinc alloy coated steels have also been developed. Coatings include zinc-iron (15 to 80% Fe) and zinc-nickel (10 to
14% Ni) alloys. These coatings are applied by electrodeposition. Zinc-iron coatings offer excellent corrosion resistance
and weldability. Zinc-nickel coatings are more corrosion resistant than pure zinc coatings, but problems include
brittleness from residual stresses and the fact that the coating is not completely sacrificial, as is a pure zinc coating. This
can led to accelerated corrosion of the steel substrate if the coating is damaged (Ref 5).
Multilayer coatings that take advantage of the properties of each layer have been developed in Europe. An example of this
is Zincrox, a zinc-chromium-chromium oxide coating (Ref 5). The CrOx top layer of this coating acts as a barrier to
perforation and provides excellent paint adhesion and weldability (Ref 5).
Another relatively new development in zinc alloy coatings is Galfan, a Zn-5Al-mischmetal alloy coating applied by hot
dipping. Applications in the United States are limited, but European automakers have used Galfan in such applications as
brake servo housings, headlight reflectors and frames, and universal joint shrouds (Ref 6). Galfan is also being considered
for oil pans, and heavily formed unexposed body panels. Detailed information is available in the article "Electroplated
Coatings" in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook.
Zinc spraying consists of projecting atomized particles of molten zinc onto a prepared surface. Three types of spraying
pistols are in commercial use: the molten metal pistol, the powder pistol, and the wire pistol. The sprayed coating is
slightly rough and slightly porous; the specific gravity of a typical coating is approximately 6.35, compared to 7.1 for cast
zinc. This slight porosity does not affect the protective value of the coating, because zinc is anodic to steel. The zinc
corrosion products that form during service fill the pores of the coating, giving a solid appearance. The slight roughness
of the surface makes it an ideal base for paint, when properly pretreated.
On-site spraying can be performed on finished parts of almost any shape or size. When applied to finished articles, welds,
ends and rivets receive adequate coverage. Moreover, it is the only satisfactory method of depositing unusually thick zinc
coatings ( ³ 0.25 mm, or 0.01 in.)
References cited in this section
1. D.J. Bologna, Corrosion Resistant Materials and Body Paint Systems for Automotive Applications (SAE
Paper 862015), in Proceedings of the Automotive Corrosion and Prevention Conference, P-188, Society of
Automotive Engineers, 1986, p 69-80
2. "US Automotive Market for Zinc Coatings 1984-1986," Zinc Institute Inc.
3. R. J. Neville and K.M. DeSouza, Electrogalvanized or Hot Dip Galvanized--Results of Five Years of
Undervehicle Corrosion Testing (SAE Paper 862010), in Proceedings of the Automotive Corrosion and
Prevention Conference, P-188, Society of Automotive Engineers, 1986, p 31-40
4. J.F.H. van Eijnsbergen, Supplement (to Twenty Years of Duplex Systems), Thermisch Verzinken, Vol 8,
1979
5. M. Memmi et al., A Qualitative and Quantitative Evaluation of Zn + Cr-CrOx Multilayer Coating Compared
to Other Coated Steel Sheets (SAE Paper 862028), in Proceedings of the Automotive Corrosion and
Prevention Conference, P-188, Society of Automotive Engineers, 1986, p 175-185
6. R.F. Lynch and F.E. Goodwin, "Galfan Coated Steel for Automotive Applications," SAE Paper 860658,
Society of Automotive Engineers, 1986
Aluminum Coatings
Aluminized (aluminum-coated) steel sheet is used for applications where heat resistance, heat reflectivity, or barrier-layer
resistance to corrosion is required. Aluminum coating of steel sheet is done on continuous lines similar to those used for
hot dip galvanizing of steel sheet. Cold-rolled steel sheet is hot dipped into molten aluminum or an aluminum alloy
containing 5 to 10% Si. The coating consists of two layers, the exterior layer of either pure aluminum or aluminum-silicon
alloy and the steel base metal, with an aluminum-iron-silicon alloy layer in between. The thickness of this alloy can
significantly affect the ductility, adhesion, uniformity, smoothness, and appearance of the surface and is controlled for
optimum properties.
Aluminum-coated sheet steel combines the desirable properties of aluminum and steel. Steel has a greater load-bearing
capacity, having a modulus of elasticity of about three times that of unalloyed aluminum. The thermal expansion of steel
is approximately half as much as that of aluminum. The aluminum coating offers corrosion resistance, resistance to heat
and oxidation, and thermal reflectivity. Typical applications are:
· Automotive mufflers and related components
· Catalytic converter heat shields
· Drying and baking ovens
· Industrial heating equipment
· Fireplaces
· Home incinerators and furnaces
· Fire and garage doors
· Kitchen and laundry appliances
· Metal buildings
· Agricultural equipment
· Silo roofs
· Playground equipment
· Outdoor furniture
· Signs, masts, and lighting fixtures
· Containers and wrappers
Coating Weight. Aluminum coatings on steel sheet are designated according to total coating weight on both surfaces in
ounces per square foot of sheet, as indicated in Table 11. These coating categories are listed in ASTM Specification A
463. Type 1, Light Coating, is recommended for drawing applications and when welding is a significant portion of the
fabrication. Type 1, Regular or Commercial, has approximately a 25 μm (1 mil) thick coating on each surface (Fig. 6a). It
is designated for applications requiring excellent heat resistance. Type 2 has a coating approximately 50 μm (2 mil) thick
on each side (Fig. 6b). It is frequently used for atmospheric corrosion resistance. Coating weight on specimens from
aluminum-coated sheet is determined by the test method in ASTM A 428. Figure 7 demonstrates how a typical rear
suspension of a front-wheel drive vehicle utilizes type 1 aluminized steel components having a coating of Al-9Si-3Fe in
conjunction with galvanized front pivot hangers, mounting brackets, and braces.
Base Metal and Formability. Aluminum coating can be applied to CQ, DQ, or DQSK steel sheet, depending on the
severity of the forming or drawing required. Only moderate forming and drawing are recommended for aluminized steel
sheet, but there are numerous intricate components for heating, combustion, and other equipment being produced.
Shallow crazing (hairline cracks) may occur in the coating if the bending and forming are too severe. To eliminate
crazing, the radius of the bend should be increased. If the crazing is deep enough to expose the steel to the atmosphere
during service, staining may occur. These stains generally have minimal effect on the serviceability of the product,
because the corrosion stops at the crazed area after a relatively short exposure period. However, if water collects and does
not drain off, corrosion products are dissolved and corrosion continues.
The mechanical properties of hot dip aluminized steel sheet are essentially the same as those of hot dip galvanized
steel sheet (Table 9). When high strength is required, SQ aluminized steel sheet may be specified, although at some
sacrifice in ductility.
Corrosion Resistance. The value of aluminum as a protective coating for steel sheet lies principally in its inherent
corrosion resistance. In most environments, the long-term corrosion rate of aluminum is only about 15 to 25% that of
zinc. Generally, the protective value of an aluminum coating on steel is a function of coating thickness. The coating tends
to remain intact and therefore provides long-term protection.
Aluminum coatings do not provide sacrificial protection in most environments, particularly in atmospheric exposure. This
is because a protective oxide film forms on the coating, which tends to passivate the aluminum and retard sacrificial
protection. If the oxide film is destroyed, the aluminum will provide sacrificial protection to the base metal. In marine or
salt-laden environments, the aluminum coating will protect sacrificially wherever chlorides destroy the surface oxide film.
Although staining or light rusting of the steel may occur at cut edges or crazing may occur where the aluminum does not
protect, this action diminishes with further exposure time because of the self-sealing action of corrosion products.
However, if insufficient slope or drainage permits water to pond or remain instead of running off freely, the corrosion
products are dissolved and rusting will continue.
Heat Resistance. Aluminum-coated sheet steel has excellent resistance to high-temperature oxidation. At surface
temperatures below about 510 °C (950 °F), the aluminum coating protects the steel base metal against oxidation without
discoloration. Between 510 and 675 °C (950 and 1250 °F), the coating provides protection to the steel, but some
darkening may result from the formation of aluminum-iron-silicon alloy. The alloy is extremely heat resistant, but upon
long exposure at temperatures above 675 °C (1250 °F), the coating may become brittle and spall because of a different
coefficient of expansion from that of the steel.
Because of their good resistance to scaling, combined with the structural strength of the steel base metal, type 1 coatings
are used in automotive exhaust systems, heat exchangers, ovens, furnaces, flues, and tubing. The higher strength of the
steel base metal, which melts at 1580 °C (2876 °F), enables steel sheet coated with either type 1 or type 2 coatings to
perform for a longer time than aluminum alone in the event of fire.
Heat Reflection. The thermal reflectivity of aluminum-coated steel sheet is comparable to that of aluminum sheet. It is
superior to galvanized steel sheet after a relatively short exposure time. All three sheet materials have thermal reflectivity
of approximately 90% before exposure. However, after a few years, the value for galvanized steel decreases more than
that for aluminized steel. Aluminum and aluminum-coated steel sheet retain 50 to 60% of their heat reflectivity. This is
advantageous where heat must be confined, diverted, or excluded, as in heat transfer applications. When used for roofing
and siding, aluminum-coated sheet keeps buildings cooler in summer and warmer in winter.
Weldability. Aluminum-coated steel sheet can be joined by electric resistance welding (spot welding or seam welding).
It can also be metal arc welded, flash welded, or oxyacetylene welded. Thorough removal of grease, oil, paint, and dirt
followed by wire brushing is recommended before joining. Special fluxes are required for metal arc and oxyacetylene
welding. During spot welding, electrodes tend to pick up aluminum, and the tips must be dressed more frequently than
during spot welding of uncoated steel. Also, higher current density is required.
Painting is generally unnecessary, but aluminum-coated sheet steel can be painted similarly to aluminum sheets. This
includes removal of oil or grease and treatment with a phosphate, chromate, or proprietary wash-type chemical before
painting.
Handling and Storage. The coating on aluminized steel sheet is soft, and care should be taken to avoid scratching and
abrasion of the soft coating, which will mar the appearance and allow staining if the coating is removed. Wet-storage
stains develop on aluminum-coated steel sheet that is continuously exposed to moisture. The sheet should be inspected for
entrapped moisture when received and then stored indoors in a warm, dry place. Some added protection can be obtained
by ordering the sheet oiled or chemically treated for resistance to wet-storage stain.
Reference cited in this section
1. D.J. Bologna, Corrosion Resistant Materials and Body Paint Systems for Automotive Applications (SAE
Paper 862015), in Proceedings of the Automotive Corrosion and Prevention Conference, P-188, Society of
Automotive Engineers, 1986, p 69-80
Aluminum-Zinc Alloy Coatings
In recent years, the desire and need to improve the corrosion resistance of galvanized coatings while retaining sacrificial
galvanic corrosion behavior at sheared edges, and so on, have led to the commercial development of two types of hot dip
aluminum-zinc alloy coatings. One type consists of about 55% Al and 45% Zn; the other type, zinc plus 5% Al. Both
coating types contain small amounts of other alloying elements to improve wettability and/or coating adhesion during
forming. Descriptions of these products are contained in ASTM A 792 (55Al-45Zn coating) and A 875 (Zn-5A1 coating).
These specifications include the general requirements, the coating categories available, and the product types that are
available.
The 55% Al coating has been produced worldwide by a number of steel companies for more than 10 years. Its primary
use is for preengineered metal buildings, especially roofing. In most environments, this coating has been found to have
two to four times the corrosion resistance of galvanized coatings while retaining an adequate level of galvanic protection
to minimize the tendency toward rust staining at edges and other breaks in the coating. Figure 8 illustrates the corrosion
resistance of 55Al-Zn-coated steel exposed to four atmospheres. The coated sheet is available in similar grades (CQ, DQ,
high strength, and so on) as hot dip galvanized and can be subjected to similar types of forming. It can also be painted
either by coil-line painting methods or postpainting after fabrication.
The coating microstructure consists of an aluminum-iron intermetallic alloy bond between the steel and outer coatingmetal
layer . This outer coating layer has a duplex microstructure, a matrix phase of an aluminum-rich
composition, and a zinc-rich interdendritic phase. This zinc-rich phase corrodes preferentially to provide the galvanic
corrosion protection. The coating contains about 2% Si, which is present in the microstructure as an elemental silicon
phase. The silicon is added only to inhibit growth of the alloy layer during the hot dip coating operation.
Although this 55% Al coating is primarily used for metal-building applications, there are a variety of other applications,
including appliances and automotive parts. It offers a level of heat-oxidation resistance intermediate between galvanized
and aluminized coatings.
The Zn-5Al coating is also produced worldwide, but it is not as commonly available as the 55% Al coating. Its primary
attribute is improved coating ductility compared to hot dip galvanized coatings.
This feature, along with a somewhat improved corrosion resistance, makes this coated-sheet product attractive for deepdrawn
parts. Also, for prepainted sheets such as roll-formed metal-building panels, the improved coating ductility
minimizes the tendency toward cracking of the paint along tension bends.
The Zn-5Al coated sheet is also available in similar grades (CQ, DQ, and so on) as hot dip galvanized. It is readily
paintable, including coil-line prepainting.
Both types of aluminum-zinc coating have features that make them more attractive than galvanized for certain
applications. Selection of either one should be based on consideration of the desired attributes and differences in
fabricability, weldability, paintability, and so on, compared to the other coatings available.
Reference cited in this section
7. H.E. Townsend and J.C. Zoccola, Atmospheric Corrosion Resistance of 55% Al-Zn Coated Sheet Steel: 13-
Year Test Results, Mater. Perform., Vol 18, 1979, p 13-20
Tin Coatings
Tin coatings are applied to steel sheet by electrolytic deposition or by immersion in a molten bath of tin (hot dip process).
Hot dip tin coatings provide a nontoxic, protective, and decorative coating for food-handling, packaging, and dairy
equipment, and they facilitate the soldering of components used in electronic and electrical equipment. In the United
States, hot dip tin coating has been replaced by electrolytic tin coating.
Electrolytic tin coated steel sheet is used where solderability, appearance, or corrosion resistance under certain conditions
is important, as in electronic equipment, food-handling and processing equipment, and laboratory clamps. It is generally
produced with a matte finish formed by applying the coating to base metal sheet called black plate, which has a dull
surface texture, and by leaving the coating unmelted. It can also be produced with a bright finish by applying the coating
to base metal having a smooth surface texture and then melting the coating. Electrolytic tin coated sheet is usually
produced in nominal thicknesses from 0.38 to 0.84 mm (0.015 to 0.033 in.) and in widths from 305 to 915 mm (12 to 36
in.).
Electrolytic tin coated steel sheet can be specified to one of the five coating-weight designations listed in Table 12. The
coating weight is the total amount of tin on both surfaces, expressed in ounces per square foot of sheet area. Electrolytic
coatings can be applied to CQ, DQ, or DQSK steel sheet, depending on the severity of the forming or drawing required.
They can also be applied to SQ steel sheet when higher strength is required. Electrolytic tin coated steel sheet is covered
in ASTM A 599. The mechanical properties of the steel sheet are unchanged by the electrolytic tin coating process.
Terne Coatings
Long terne steel sheet is carbon steel sheet continuously coated by the hot dip process with terne metal (lead with 3 to 15
wt% Sn). This coated sheet is duller in appearance than tin-coated sheet, hence the name (terne) from the French, which
means dull or tarnished. The smooth, dull coating gives the sheet corrosion resistance, formability, excellent solderability,
and paintability. The term long terne is used to describe terne-coated sheet, while short terne is used for terne-coated
plate. Short terne, also called terneplate, is no longer produced in the United States.
Because of its unusual properties, long terne sheet has been adapted to a wide variety of applications. Its greatest use is in
automotive gasoline tanks. Its excellent lubricity during forming, solderability and special corrosion resistance make the
produce well suited for this application. Other typical applications include:
· Automotive parts, such as air conditioners, air filters, cylinder head covers, distributor tubes, oil filters,
oil pans, radiator parts, and valve rocker arm covers
· Electronic chassis and parts for radios, tape recorders, and television sets
· File drawer tracks
· Fire doors and frames
· Furnace and heating equipment parts
· Railroad switch lamps
· Small fuel tanks for lawn mowers, power saws, tractors, and outboard motors
Long terne sheet is often produced to ASTM A 308. The coatings are designated according to total coating weight on both
surfaces in ounces per square foot of sheet area, as indicated in Table 13. For applications requiring good formability, the
coating is applied over CQ, DQ, or DQSK low-carbon steel sheet. The terne coating acts as a lubricant and facilities
forming, and the strong bond of the terne metal allows it to be formed along with the base metal. When higher strength is
required, the coating can be applied over SQ low-carbon steel sheet, although there is some sacrifice in ductility. In
general, the mechanical properties of hot dip terne-coated steel are similar to those for cold-rolled steel. Terne coatings
are applied by a flux-coating process that does not include in-line annealing. Therefore, the mechanical properties are
obtained by pre-annealing using cycles comparable to those used for cold-rolled sheet.
Lead is well known for its excellent corrosion resistance, and terne metal is principally lead, with some tin added to form
a tight, intermetallic bond with steel. The excellent corrosion resistance of terne sheet accounts for its wide acceptance as
a material for gasoline tanks. However, because lead does not offer galvanic protection to the steel base metal, care must
be exercised to avoid scratches and pores in the coating. Small openings may be sealed by corrosion products of iron,
lead, and oxygen, but larger ones can corrode in an environment unfavorable to the steel base metal.
Long terne sheet can be readily soldered with noncorrosive fluxes using normal procedures because the sheet is already
presoldered. This makes it a good choice for television and radio chassis and gasoline tanks, for which ease of
solderability is important. It can also be readily welded by either resistance seam or spot welding methods. However,
when the coating is subjected to high temperatures, significant concentrations of lead fumes can be released. Because of
the toxicity of lead, the Occupational Safety and Health Administration and similar state agencies have promulgated
standards that must be followed when welding, cutting, or brazing metals that contain lead or are coated with lead or lead
alloys.
Long terne sheet has excellent paint adherence, which allows it to be painted using conventional systems, but this product
is not usually painted. When painting is done, no prior special surface treatment or primer is necessary, except for the
removal of ordinary dirt, oil, and grease. Oiled sheet, however, should be thoroughly cleaned to remove the oil.
Long terne sheet is normally furnished dry and requires no special handling. It should be stored indoors in a warm, dry
place. Unprotected, outdoor storage of coils or bundles can result in white or gray staining of the terne coating, and if
there are pores in the terne coating, rust staining can occur.
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Aldebaran
 
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