GB2230477A

GB2230477A - Method for enamelling a plate metal object and the object obtained

Info

Publication number: GB2230477A
Application number: GB9008099A
Authority: GB
Inventors: Antonius Wilhelmus Cornel Dijk
Original assignee: Ferro Tech
Current assignee: Ferro Tech
Priority date: 1989-04-18
Filing date: 1990-04-10
Publication date: 1990-10-24
Anticipated expiration: 2010-04-10
Also published as: DE4011932A1; NL194207C; BE1004927A3; NL194207B; GB9008099D0; DE4011932C2; NL9000423A; GB2230477B

Description

P HP/LS/ferro-3p 1 METHOD FOR ENAMELLING A PLATE METAL OBJECT AND THE

OBJECT OBTAINED The present invention relates to a method for enamelling a plate metal object and to an object obtained.

Many metal objects are subjected to a surface coating with enamel, which has a good chemical resistance and is fairly insensitive to thermal (shock) loadings. In addition enamel is not moisture permeable and displays no underrusting. A drawback of enamel is however that it is not impact resistant, that only by applying multiple layers does it display a substantially pore-free surface and that it is susceptible to so-called fish-scale forming. Fish-scale forming is the consequence of accumulation of molecular hydrogen at the interface between the metal and the enamel coating. such a high gas pressure can herein occur that the adhesion strength of the enamel to the metal and the break- ing strength of the enamel coating is exceeded, blisters result and pieces of enamel coating even break off.

Another type of coatings for metal surfaces are paints or lacquers, such as epoxy lacquers, isocyanate lacquers, polyurethane lacquers and silicon resin lacquer, made up of different components. These lacquers have a good chemical resistance, are gas permeable, whereby they display no fishscale forming, but have the drawback of underrusting.

The invention has for its object to provide a method for enamelling a plate metal object, wherein the drawbacks of the use of enamel, vulnerability to shock loads, and the drawbacks of the use of lacquer, underrusting, are avoided, but where furthermore an object results which has an acceptable visual appearance. This is achieved according to the invention with a method for enamelling a plate metal object comprising the steps of:

i) pre-treating the plate metal object; 2 applying a cermet enamel coating to a pre-treated side; iii) fusing the applied enamel coating at suitable fusing temperatures; and iv) applying a gas permeable, chemically resistant lac- quer coating on the cermet enamel coating.

If the lacquer is applied using the wet method it can be applied directly onto the cermet enamel coating. If however the lacquer is applied via a dry method, powder coating for instance, it is then preferable to apply between the lacquer coating and the cermet enamel coating an intermediate layer which ensures a sufficient adhesion between the lacquer coating and the cermet enamel coating. Through the use of the external lacquer layer the exterior of the plate metal object can be given any desired colour.

The drawback of fish-scale forming occurs particularly in two-sided enamelling of plate meatal objects, since with such a sandwich enamel coating hydrogen is enclosed in the metal between both the enamel coatings and can hardly or not diffuse away to the sides in the metal layer.

The invention also has for its object to provide for this application an improvement for two-sided enamelling of a plate metal object, wherein the drawback of fish-scale forming substantially does not occur, a good chemical resis- tance, moisture impermeability and the non-occurrence of underrusting are preserved, while the impact resistance is improved and, if required, repair of the applied coating is possible.

According to a preferred embodiment, in step ii) an enamel coating is applied to two pre-treated sides of the plate metal object wherein on at least one side a cermet enamel coating is applied as enamel coating.

The laminate of a laminate coating consisting of a substrate of a specific type of enamel, socalled cermet enamel, and a top layer of the above described coating lacquers fulfills these requirements. This laminate does not display any fish-scale forming, because both the cermet 1 3 enamel coating and the lacquer coating are gas permeable, wherein the lacquer layer provides the chemical resistance for the laminate and the cermet enamel coating provides the adhesion to the metal surface and the resistance to under5 rusting.

With the method according to the invention it is unnecessary for both sides of the plate metal object to be provided with a laminate according to the invention. It is in principle only necessary that such a laminate be applied to one side of the plate metal object, since the hydrogen responsible for the eventual fish-scale forming can diffuse from a side coated with a gas impermeable enamel to the other side which is coated with a gas permeable coating according to the invention and can escape therethrough to the atmosphere. This affords the possibility of using as top layer for a plate metal object a chemically resistant, gas tight enamel with a specific function (anti-sludge enamel), while the underside according to the invention is gas per meable and the whole object displays no fish-scale forming.

if the laminate built up of the cermet coating and the lacquer coating is present on both sides of the plate metal object, both lacquer coatings can consist of different materials, whereby on the inside a chemical (acid) resis tance occurs, and on the outside a lesser chemical (weather) resistance occurs. This application is for example suitable for silos for manure storage, feed storage and for tanks in water purification.

Another aspect of the invention relates to a plate metal, enamelled object which comprises:

i) a metal base; ii) a cermet enamel coating on at least one side; and iii) a lacquer coating applied to the cermet enamel coat ing.

In the case of two-sided enamelling the object com- prises:

i) a metal base with two sides; ii) an enamel coating on each side, of which at least one 4 enamel coating is a cermet enamel coating; and iii) a lacquer coating applied to the cermet enamel coat- ing.

An enamel is a glassy mass of inorganic, substantial- ly oxides-comprising composition resulting from melting or so-called fritting which can be applied and fused in one or more layers onto metal or glass objects. Typical additives for enamelare silicon oxide, boron oxide and phosphorous oxide which are so-called network forming agents. Alkaline and alkaline earth metals can further be added, in addition to oxides of cobalt, nickel, copper and antimony. Diverse enamel types are well known to persons skilled in the art as are their compositions. Reference is made to the enamel literature for further information.

A very specific type of enamel is the cermet enamel.

This enamel contains finely distributed metal particles, for instance aluminium particles. For more information concern ing cermet enamel reference is made to I'Mitteilungen des Vereins deutscher Emailfachleute E.V.11, part 27, pages 49-60 (1979) and part 28, pages 13-24 (,1980).

The suitable organic lacquer coating can consist of a known lacquer system, such as physically drying or chemical ly curing, optionally through thermal treating. The type of lacquer coating used is geared to the required properties.

Epoxy or polyurethane lacquers give for instance a high chemical resistance, acrylate lacquers possess a great resistance to weather influences and are colourfast, silicon lacquers have a great heat resistance. A lacquer can thus be selected depending on the degree of chemical resistance, such as acid resistance, weather resistance and the like.

Various types of steel can be used as metal, in particular hot-rolled steel that is extremely susceptible to fish-scale forming.

A plate metal object, for instance a silo wall, is pre-treated on both sides prior to enamelling. These pretreatments can consist of heating to red-heat and radiation, degreasing-pickling and optionally nickel-plating and, fi- 1 nally, drying. The cermet enamel can be applied by means of spraying or immersion and after drying is fused at a suit able fusing temperature, for instance approximately 850'C, this depending on the specific composition of the cermet enamel used. Finally, the gas permeable, chemically resis tant lacquer coating is applied to the fused cermet enamel coating by means of spraying or immersion, whereafter the lacquer coating is cured by muffling.

Following hereinafter are examples of the use of plate metal objects which are enamelled on two sides accord ing to the invention.

EXAMPLE I

For a floor grating in a farm building lay-out it is impor tant that the top side of the floor grating is provided with is an anti-sludge layer. In order to avoid fish-scale forming it can suffice to apply to the underside of the floor grat ing a laminate according to the invention which consists of a substrate of a cermet coating and a gas permeable, chemi cally resistant lacquer coating arranged thereon. After a suitable pre-treatment a conventional base enamel (Bayer GR1100/GR1200, GR1308/GR1400) is thus applied to the top side of the floor grating and a cermet enamel layer to the underside. Both enamel layers are fused simultaneously at approximately 8500C. An anti-sludge enamel (Ferro Holland 6044/XS50) is then applied to the fused base enamel coating and subsequently fused. The underside is then provided with an epoxy lacquer layer (Ferro Topcoat from Delta Holdings) which is subsequently muffled. The final floor grating ful fills the demands required, namely no fish-scale forming through use of the laminate according to the invention, no underrusting through use of base enamel and cermet enamel as bottom coating, chemical resistance through use of the anti sludge enamel layer on the top side and the lacquer coating on the underside.

In similar manner the interior parts and components of heaters and central heating boilers can be provided with the cermet enamel coating which is finally covered with a 6 silicon resin lacquer. EXAMPLE II In the following experiments 1-5 enamel mixtures are tested for suitability in manure silos, wherein as test parameters are used: pore formation, fish-scale forming and chemical resistance.

The tested steel types are: - steel E560 Impress, with a carbon content of 0.04-0.07% in an alloy with niobium, and hot-rolled; and - steel ST37 in normal trade quality, known on account of fish-scale susceptibility, and hot-rolled.

Used on both these steel types are five different enamel mixtures: V = enamel ceptible c frit 2290 from Ferro Holland, a fish-scale susenamel with the following composition:

Li,0 0 - 5% Na,0 10 - 15% K,0 0 - 5% CaO 0 - 10% B,O, 5 - 10% Sio, 30 40% TiO, 5 - 10% C0,0, 0 - 5% NiO 0 - 5% G base enamel (Ferro Holland fish-scale resistance having tion (% by weight):

2215/2218/2220 (50/25/25) Na,0 15 - 20 K,0 2 - 10 CaO 8 - 13 BaO 0 - 5 Sio, 30 - 45 B,O, 10 - 20 A1,0, 0 - 5 Plor, 1 - 4 coo 0.5 -2 2215/2216/2218) with a high as oxides-comprising composi 1 7 1 NiO 1 - 3 CUO 0 - 2 ZnO 0 - 4 Fe,O, 0 - 5 F 0 - 8 R = acid resistant direct enamel (Bayer GS 1533/AC8535-10/ AC8535-11), suitable for enamelling heat exchanger plates; D = top enamel (Ferro Holland 6044) with a very high acid resistance having as oxides-comprising composition: 6044 C = cermet 0 5 15 20 45 65 2 8 3 6 Li,0 Na,0 Sio, B,O, TiO, SrO 0 - 5 Zr02 5 - 15 moo, 0 - 2 F 0 - 10; and enamel (Ferro Holland 0904/0912/0916), an enamel enriched with aluminium and possessing a high heat resis tance and gas permeability.

The different enamel coatings are applied to the front and rear sides and optionally fused simultaneously, whereafter a second enamel coating is applied to the front side and optionally the rear side.

The experiments 2-5 are according to the invention, because as outer enamel coating a cermet coating is applied which is subsequently covered with a lacquer coating.

Pore formation is examined according to NEN2709 me thod Bl, while the fish-scale forming is examined with test enamel 2290 from Ferro Holland by evaluating fish-scale forming after enamelling and arranging for 24 hours at 60 800C.

The results are summarized in the following table.

1 1 1 1 1 1 1 1 1 ------ 1 1Exp.1steel iFronti Rear 1FrontiRear 1PoresiFish- 1RemarksI 1 Itype iside J side Iside Iside IfrontIscale 1 1 11st 11st 12nd 12nd Iside Ifront 1 1 Ilayerllayer Ilayerllayerl Iside 1 1 1 E560 V V St37 V V 2 E560 G G 1 R 1 C Inone St37 G G 1 R 1 C Inone 1 1 3 1 E560 IG/R+ 1 C 1 R 1 C Inone 1 St37 1 1 1 1 1 4 E560 G C R St37 E560 G C D St37 G C D 1 yesl afterl 1 yes, 1 16 hrsl 1 11 dir. 1 no no 1 no 1 1 1 none 1 no 1 none 1 no 1 none no : after 16 hours 1: directly The objects coated according to the invention were also found to possess a good chemical resistance.

EXAMPLE III

A steel plate (ST37) was coated on both sides with a cermet enamel coating of the following composition:

LiO, 3 - 6% Na,0 10 - 16% K,0 1 - 4% CaO 1 - 4% BaO 0 - 1% 1 t 9 MnO coo NiO CUO B,O, Fe,O, Sio, TiO, F 2 4% 1 3% 0 1% 9 18% 0 5% 60% 1 3% 0 4% The enamel sludge is prepared in accordance with the grinding formulation:

cermet enamel (see above) quartz clay boric acid borax sodium nitrite aluminium powder parts parts 6 parts 0.3 parts 0.3 parts 0.2 parts parts water 60 parts Applied to the one (inner) side of the plate was a lacquer coating of an aliphatic isocyanate lacquer (Ferrodur Resistence from Delta Coatings) consisting of three com- ponents, and on the other (outer) side a polyurethane lacquer (Ferrodur Topcoating from Delta Coatings) consisting of two components. The isocyanate lacquer possesses a high chemical resistance, while the polyurethane lacquer has an adequate weather resistance.

These plates showed no pores or fish-scale forming in the respective pore and fish-scale tests. These plates were subsequently subjected to a corrosion test which is a variant of the Kesterinich test, wherein the samples were tested intermittently. A cycle consisted of eight circuits, wherein the test plates were exposed for 8 hours to an atmosphere with a relative humidity of 100% and 2 litres of S02 at a temperature of 40'C, whereafter the test samples were exposed to air for 16 hours. After five circuits the test samples were exposed for 36 hours to an atmosphere with a relative humidity of 100% and 0.5 litres of ammonia at a temperature of 20'C.

After four cycles of a week the S02 concentration was increased to 3 litres and the temperature was varied as follows: 40'C for 4 hours, followed by 3 hours of cooling to 20'C and 1 hour of heating to 40C. The test samples were evaluated for rust formation, blister forming (ASTM D71456). The inner side of the test plates covered with an iso- cyanate lacquer were found to possess an excellent chemical resistance, while the inner side of the test plates coated with polyurethane lacquer were found to have a good weather resistance. These laminate coatings, consisting of an undercoating of cermet enamel and a top coating of gas permeable, chemically resistant lacquer, can thus be used for manure silos and the like. EXAMPLE IV A formed steel plate is coated on one side with a cermet enamel coating as in any of the foregoing examples I- 111. Applied to the fused cermet enamel coating by powder spraying or immersion is an intermediate layer (Betoran Super Primer VV from Delta Coatings) which is dried. A powder coating (Wbrwag) is subsequently applied by electrostatic powdering. Both layers are then muffled simultaneous25 ly at a muffling temperature of 180-200'C. This plate coated according to the invention can be used in radiators that are arranged in very humid areas such as swimming pools. With the epoxy powder used as lacquer layer the radiator acquires for instance a white colour. The radiator is optimally protected against rust and at the same time the vulnerability to mechanical shock loadings is lessened.

11

Claims

1. Method for enamelling a plate metal object comprising the steps of:

i) pre-treating the plate metal object; ii) applying a cermet enamel coating to a pre-treated side; iii) fusing the applied enamel coating at suitable fusing temperatures; and iv) applying a gas permeable, chemically resistant lac- quer coating on the cermet enamel coating.

2. Method as claimed in claim 1, wherein an inter- mediate layer is applied between the lacquer coating and the cermet enamel coating.

3. Method as claimed in claim 1 or 2, wherein in step ii) an enamel coating is applied to two pre-treated sides of the plate metal object, wherein on at least one side a cermet enamel coating is applied as enamel coating.

4. Method as claimed in claim 3, wherein in step ii) a cermet enamel coating is applied to both sides.

5. Method as claimed in claim 4, wherein both cermet enamel coatings are provided with a gas permeable, chemical- ly resistant lacquer coating.

6. Method as claimed in claim 3, wherein both lacquer coatings have a mutually differing chemical resistance.

7. Method as claimed in claims 1-3, wherein the ena mel coating is provided with a chemically resistant enamel coating.

8. Method as claimed in claims 1-7, wherein the lac quer coating comprises an epoxy or polyurethane lacquer, an acrylate lacquer, a silicon lacquer or mixtures thereof.

9. Plate-like, enamelled object comprising:

i) a metal base; ii) a cermet enamel coatina on at least one side; and iii) a lacquer coating on the cermet enamel coating.

10. A plate-like object as claimed in claim 9, comprising an intermediate layer between the cermet enamel coating and the lacquer layer.

11. A plate-like object as claimed in claim 9 or claim 10, wherein the metal base has an enamel coating on each of its two sides, at least one enamel coating being a cermet enamel coating and bearing a lacquer coating.

12. A plate-like object as claimed in claim 11, wherein each enamel coating is a cermet enamel coating.

13. A plate-like object as claimed in claim 12, wherein each cermet enamel coating bears a lacquer coating.

14. A plate-like object as claimed in any of claims 9 to 13, wherein the enamel coating has a chemicallyresistant enamel coating.

15. A plate-like object as claimed in any of claims 9 to 14, wherein the or each lacquer coating comprises an epoxy or polyurethane lacquer, an acrylate lacquer, a silicone lacquer, or a mixture thereof.

16. A method as claimed in claim 1, substantially as described in any of the Examples.

17. A plate- like object obtained by a method as claimed in any of claims 1 to 8 and 16.

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