Preacoted Steel Sheet (2)

Moderatore: Moderatore Area Tecnica

Preacoted Steel Sheet (2)

Sostienici

Sponsor
 

Preacoted Steel Sheet (2)

Messaggioda Aldebaran » 10/05/2010, 13:42

Phosphate Coatings
The phosphate coating of iron and steel consists of treatment with a dilute solution of phosphoric acid and other chemicals
by which the surface of the metal, reacting chemically with the phosphoric acid, is converted into an integral layer of
insoluble crystalline phosphate compound. This layer is less reactive than the metal surface and at the same time is more
absorbent of lubricants or paints. Because the coating is an integral part of the surface, it adheres to the base metal
tenaciously. The weight and crystalline structure of the coating, as well as the extent of penetration of the coating into the
base metal, can be controlled by the method of cleaning before treatment, the method of applying the solution, the
duration of treatment, and the changes in the chemical composition of the phosphating solution.
The two types of phosphate coatings in general use are zinc phosphate and iron phosphate. Within each type, chemical
composition can be modified to suit various applications.
When zinc phosphate coatings are mill applied to galvanized sheets, the sheets are ready for immediate painting with the
many paints readily available from industrial and retail suppliers. The zinc phosphate coated product is often referred to
as phosphatized. Minor cleaning with mineral spirits, paint thinner, or naphtha may be necessary to remove fingerprints,
oils, or dirt picked up during fabrication or handling. When mill-phosphatized sheets that are to be baked after painting
are exposed to humid storage conditions for long periods of time, prebaking for several minutes at 150 °C (300 °F) prior
to painting may be required to prevent blistering during baking.
The chief application for iron phosphate coatings is as a paint base for uncoated carbon steel sheet. Such a coating can be
applied on coil coating lines.
The greatest tonnage of phosphate-coated steel is low-carbon flat-rolled material, which is used for applications such as
sheet metal parts for automobiles and household appliances. Applications of the coatings range from simple protection to
prepaint treatments for painted products, such as preengineered building panels and the side and top panels of washing
machines, refrigerators, and ranges.
Phosphate coatings require a clean surface. The cleaning stage preceding phosphating removes foreign matter from the
surface and makes uniformity of coating possible. This involves removal of oils, greases, and associated dirt by solvent
degreasing or alkaline cleaning followed by thorough rinsing. Phosphate coatings are applied by spray, immersion, or
roller coating.
A phosphate coating beneath a paint film helps prevent moisture and other corrosives that may penetrate the paint from
reaching the metal. This prevents or delays the electrochemical reactions that lead to corrosion or rust. If the paint film
sustains scratches or damage that exposes bare metal, the phosphate coating confines corrosion to the exposed metal
surface, preventing the corrosion from spreading underneath the paint film. In painting applications, coarse or heavy
phosphate coatings may be detrimental; they can absorb too much paint, thus reducing both gloss and adhesion, especially
if deformation of the painted sheet steel is involved.
Preprimed Sheet
Primer paint coats are frequently applied to steel sheet at the mill or by a coil coater. Because their purpose is corrosion
protection, they contain corrosion-inhibiting substances such as zinc powder, zinc chromate, or other compounds of zinc
and/or chromium. Preprimed sheets are especially useful for parts that will have limited access after fabrication, rendering
coating difficult. Parts made from preprimed sheet may receive a top coat after fabrication. The mill-applied phosphate
coatings described in the previous section can also be considered prepriming treatments.
Formability (Ref 8). Preprimed steel offers advantages in forming metal fabrication through:
· Consistent surface morphology
· Reduced surface friction (reducing the flow over die surfaces) and reduced die wear, especially on the
binder surfaces
· Reduced flaking and powdering (requiring less die maintenance), reduced need for metal finishing, and
fewer surface defects
· Reduced galling
The painted surface acts as a cushion between substrate and stamping dies, which lessens the need for in-die lubrication
and extends the life of the stamping die. The preprimed, prepainted surface can withstand severe forming and stretching.
Thus, the need for lubricant is reduced or eliminated. This is turn provides a clean process environment and reduces the
need for extensive cleaning along with phosphating and electrocoating.
Zinc Chromate Primers. Zinc chromate pigments are useful as corrosion inhibitors in paint. They are used as afterpickling
coatings on steel and in primers. Federal specifications TT-P-57 and TT-P-645 cover zinc chromate paints. Zinc
chromate pigments are unique; they are useful as corrosion inhibitors for both ferrous and nonferrous metals.
Zinc-Rich Primers. In recent years, manufacturers have developed various priming paints that will deposit films
consisting mainly of metallic zinc that have many properties in common with the zinc coatings applied by hot dip
galvanizing, electroplating, metal spraying, or mechanical plating methods. Such films will provide some degree of
sacrificial protection to the underlying steel if they contain 92 to 95% metallic zinc in the dry film and if the film is in
electrical contact with the steel surface at a sufficient number of points. The type of zinc dust used in protective coatings
is a heavy powder, light blue-gray in color, with spherically shaped particles having an average diameter of approximately
4 μm (160 μin.). Such powder normally contains 95 to 97% free metallic zinc with a total zinc content exceeding 99%.
Many zinc-rich paints are air drying, although oven-curable primers containing a high content of zinc dust are available.
Depending on the nature of the binder, zinc-rich primers are classified as inorganic or organic. The inorganic solvent-base
types are derived from organic alkyl silicates, which, upon curing, become totally inorganic. The organic zinc-rich
coatings are formed by using zinc dust as a pigment in an organic binder. This binder may be any of the well-known
coating vehicles, such as chlorinated rubber or epoxy. The zinc dust must be in sufficient concentration so that the zinc
particles are in particle-to-particle contact throughout the film. In this way, zinc provides cathodic protection to the base
metal. With the organic binder, there is no chemical reaction with the underlying surface, but the organic vehicle must
wet the surface thoroughly to obtain mechanical adhesion.
The inorganic zinc coating forms its film and adheres to the steel surface by quite different means. The chemical activity
during coating is quite similar for either water-or solvent-base inorganic binders. Zinc is the principal reactive element in
the inorganic systems and is primarily responsible for the development of initial insolubility.
Zinc-rich primers offer a more versatile application of zinc to steel than galvanizing. Large, continuous, complex shapes
and fabricated new or existing structures can be easily coated at manufacturing shops or in the field. The performance of
zinc-rich primers has earned them a prominent place in the field of corrosion protection coatings. For example, zinc-rich
primers are being preapplied to steel sheet as the first coat of a two-coat system for appliance applications such as
refrigerator liners. However, the limitations of zinc-rich paints include cost and the required cleanliness of steel surfaces.
They must be top coated in severe environments (pH under 6.0 and over 10.5).
The following comparisons should be helpful in selecting the binder system that is most suitable for an application.
Inorganics have superior solvent and fuel resistance. They can withstand temperatures to 370 °C (700 °F) and are much
easier to clean up after use. Inorganics do not blister upon exposure and are unaffected by weather, sunlight, or wide
variations in temperature. They do not chalk, peel, or lose thickness over long periods of time. Also, they are easier to
weld through and have excellent abrasion resistance and surface hardness. Organics use chlorinated rubber, epoxy, vinyl,
phenoxy, or other coating vehicles, and the properties of the system are based on the characteristics of the vehicle used.
Reference cited in this section
8. B.K. Dubey, Prepainted Steel for Automotive Application, in Corrosion-Resistant Automotive Sheet Steels,
L. Allegra, Ed., Proceedings of a Conference in conjunction with the 1988 World Materials Congress
(Chicago), Sept 1988, ASM INTERNATIONAL, 1988
Organic Composite Coatings
Organic composite coated steels have been developed mainly by Japanese steelmakers in cooperation with automakers in
that country, although development is underway in other countries as well. These coil coated products generally employ
an electroplated zinc alloy base layer and a chemical conversion coating under a thin organic topcoat containing a high
percentage of metal powder (Ref 9, 10, 11). The thinness of the organic topcoat allows for good formability without the
risk of damaging the coating.
Figure 10 compares the corrosion resistance of one of these organic composite coated sheet steels to cold-rolled steel and
to Zincrometal. Another of these products uses an organic-silicate composite topcoat only about 1 μm (40 μin.) thick and
has corrosion resistance and weldability superior to that of Zincrometal (Ref 10). A bake-hardenable version of this
material has also been developed (Ref 10). Researchers at a third Japanese steel company have developed a bakehardenable
organic composite coated sheet steel with a 0.8 to 1.5 μm (32 to 60 μin.) thick organic topcoat. The material
possesses corrosion resistance, formability, and weldability equivalent to that of Zincrometal-KII, which uses a 7 μm (280
μin.) thick topcoat (Ref 11). Production of these composite-coated materials is increasing in anticipation of increased
demand from Japanese automakers.
A similar material has been developed in the United States. This material has an electrodeposited zinc alloy base coat, a
mixed intermediate layer of chromium oxide and zinc dust, and an organic topcoat for barrier protection (Ref 12). Figure
11 is a micrograph showing the cross section of the composite-coated steel. In salt spray tests comparing this material to
electrodeposited zinc-nickel and Zincrometal, zinc-nickel failed after 216 h, Zincrometal at 480 h, and the composite
coating at 960 h (Ref 12). This material was developed to have weldability, formability, and adhesive compatibility
similar to that of Zincrometal. Developmental work continuing.
Organic-Silicate Composite Coatings (Ref 13), Zinc-nickel electroplated steel sheet coated with an organic-silicate
composite was developed by a Japanese steel company in an attempt to combine a highly corrosion resistant base zincnickel
coating with a protective surface layer to prolong the coating life. With a view to forming a thin film with high
corrosion resistance, the protective layer was designed as a two-layer protective film structure composed of a chromate
film as a lower layer and the organic-silicate composite coating (the composite resin) as an upper layer. This protective
film structure improves the corrosion resistance not only by the individual effects of each layer, such as the passivation of
chromate film and the excellent corrosion resistance of the composite resin, but also by the suppression of excessive
dissolution of Cr6+ from the lower chromate film layer by the sealing effect of the upper composite resin layer. This
sealing effect sustains the passivation of chromate film more effectively in the corroding environment.
Prepainted Sheet
Prepainted steel sheet is a large and rapidly expanding market. The sheet is coated in coil form in a continuous coilpainting
facility. Lower production costs, improved product quality, elimination of production hazards in the shop,
customer satisfaction, conservation of energy, elimination of ecological problems, and the ability to expand production
without capital expenditure for new buildings and equipment are some of the advantages of prepainted sheet over
postpainting. Fabricated parts are readily joined by indirect projection welding, adhesives, tabs, and fasteners. Typical
applications of prepainted steel sheet include tool sheds, preengineered buildings, swimming pools, automobiles, lighting
fixtures, baseboard heaters, truck vans, mobile homes, home siding, metal awnings, air conditioners, freezer, refrigerators,
ranges, washers, and dryers.
Selection of Paint System. A wide variety of paint systems are available on prepainted sheet. In selecting the proper
system for a particular application, the user must consider fabrication requirements, the service life desired, and the
service conditions, that will be encountered. For example, in an aggressive environment a plastisol coating may be
required. For a deep draw, a vinyl coating should be used instead of a polyester. For resistance to fading in sunlight, a
silicone polyester may be suggested instead of a polyester or a vinyl paint.
In the preengineered building field, the paint system must be capable of being roll formed and still perform over the years
under a wide variety of conditions without chalking, fading, cracking, or blistering. In the automotive field, the drawing
properties of the coating must be considered in addition to corrosion protection from road salts. In the appliance industry,
a high-gloss finish that will bend without cracking is important, along with resistance to such materials as detergents,
solvents, mustard, ketchup, shoe polish, grape juice, and grease. Other product requirements frequently considered when
selecting an appliance paint are color, hardness, adhesion, resistance to abrasion, corrosion, humidity, heat, and pressure
marking.
For severe corrosion service and decorative effects, heavier coatings are supplied, often by laminating or bonding a solid
film to the metal substrate. Typical applications include buildings, roofing and siding near pickling tanks, chemicals and
other corrosive environments, and storm drains and culverts that are subjected to corrosive soils, mine acids, sewage, and
abrasion. These culvert coatings can be a thermoplastic coal tar-base laminate 0.3 to 0.5 mm (0.012 to 0.020 in.) thick, or
they can be a film of polyvinyl chloride.
Design Considerations. In using prepainted sheet, design should be considered. If necessary, binding radii, location
of exposed edges, fastening methods, welding techniques, corner assembly, and other features should be modified to
make them compatible with the base metal and paint system. For example, if a polyester paint is applied to bare steel
sheet, a minimum bend radius of 3.2 mm ( 1
8
in.) is suggested to minimize cracking and crazing of the paint. If hot dip
galvanized sheet is the substrate, of minimum bend radius of 6.4 mm ( 1
4
in.) should be used instead. Otherwise, the zinc
coating may crack with sharper bending, and the paint may not be elastic enough to bridge the crack.
Paint is often cured at temperatures as high as 240 °C (465 °F). At the higher paint curing temperatures, the steel sheet
may become fully aged and cause yield point elongation to return. The sheet is subject to the formation of stretcher strains
during subsequent forming. Normally, return of yield point elongation is not objectionable in these applications.
However, the formed part will sometimes be given a critical amount of strain, and strain lines may become visible.
Frequently, this problem can be overcome by proper shop practices, particularly if the part has been roll formed. At times,
however, it is necessary to use killed steels, which are considered essentially nonaging.
Shop Practices. Because a prepainted surface is composed of an organic material, the abuse that this surface can
withstand is less than that of a metal sheet surface. Therefore, prior to using prepainted sheet for the first time, it is
advisable to train shop personnel in proper handling practices and to examine shop equipment to eliminate sources of
scratches. For example, dies, brake presses, and roll-forming equipment must have highly polished surfaces free of
gouges, score marks, and so on. Clearances of the dies must be such that wiping of the paint film is avoided. Similarly,
some care is needed when formed parts are put on carts or in containers for transfer from one location to another. It is not
acceptable simply to pile one part on top of another. Good housekeeping is important to minimize the source of scratches.
Frequent reexamination of shop equipment and parts containers is necessary to minimize scratches. Handling scratches
can be refinished by retouching, which is costly and time consuming.
Packaging and Handling. Shop and field conditions should be considered when selecting packaging for prepainted
sheet. Transit pickoff and job-site corrosion from entrapped moisture can be serious problems. For preengineered building
sheets, for example, packaging after roll forming should include waterproof paper (no plastic wrapping), support sheets to
prevent sagging, and pressure boards. Mixing sheets of different lengths in the bundle should be avoided. Once the bundle
of formed prepainted sheets arrives at the job site, it should be inspected to determine if the packages are still intact and
resistant to the weather.
Wherever possible, sheets should be erected on the day of delivery, or they should be protected from water condensation.
Under-roof storage is desirable. However, if this is not possible, the waterproof bundles should be slanted so that any
condensation will drain out. Damaged packages should be opened, inspected, and the sheets separated to allow complete
drying. In addition to the prevention of moisture entrapment described above, chips from drilling operations should be
brushed away to prevent rust spotting. Prepainted sheets should be installed with corrosion-resistant fasteners. The
installation of sheets that are in contact with the soil should be avoided. Oil, grease, fingerprints, and other contaminants
should be removed after installation.
References
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
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
8. B.K. Dubey, Prepainted Steel for Automotive Application, in Corrosion-Resistant Automotive Sheet Steels,
L. Allegra, Ed., Proceedings of a Conference in conjunction with the 1988 World Materials Congress
(Chicago), Sept 1988, ASM INTERNATIONAL, 1988
9. Y. Shindou et al., Properties of Organic Composite-Coated Steel Sheet for Automobile Body Panels (SAE
Paper 862016), in Proceedings of the Automotive Corrosion and Prevention Conference, P-188, Society of
Automotive Engineers, 1986, p 81-90
10. M. Yamashita, T. Kubota, and T. Adaniya, Organic-Silicate Composite Coated Steel Sheet for Automobile
Body Panel (SAE Paper 862017), in Proceedings of the Automotive Corrosion and Prevention Conference,
P-188, Society of Automotive Engineers, 1986, p 91-97
11. T. Mohri et al., Newly Developed Organic Composite-Coated Steel Sheet With Bake Hardenability (SAE
Paper 862030), in Proceedings of the Automotive Corrosion and Prevention Conference, P-188, Society of
Automotive Engineers, 1986, p 199-208
12. T.E. Dorsett, Development of a Composite Coating for Pre-Coated Automotive Sheet Metal (SAE Paper
862027), in Proceedings of the Automotive Corrosion and Prevention Conference, P-188, Society of
Automotive Engineers, 1986, p 163-173
13. T. Watanabe, T. Kubota, M. Yamashita, T. Urakawa, and M. Sagiyama, Corrosion-Resistant Precoated
Steel Sheets for Automotive Body Panels, in Corrosion-Resistant Automotive Sheet Steels, L. Allegra, Ed.,
Proceedings of a Conference in conjunction with the 1988 World Materials Congress (Chicago), Sept 1988,
ASM INTERNATIONAL, 1988
Avatar utente
Aldebaran
 
Messaggi: 5460
Images: 5001
Iscritto il: 31/03/2010, 19:26
Località: Milano - Chieti

Torna a Iron & Steel

Chi c’è in linea

Visitano il forum: Nessuno e 1 ospite