GB2189409A

GB2189409A - Protective coatings for metals

Info

Publication number: GB2189409A
Application number: GB08709031A
Authority: GB
Inventors: Robert Robertson; David Martin Hoult
Original assignee: Cannon Industries Ltd
Current assignee: Cannon Industries Ltd
Priority date: 1986-04-18
Filing date: 1987-04-15
Publication date: 1987-10-28
Also published as: GB8709031D0; GB8609498D0

Abstract

Metal vessels, especially those used in chemical plants, may be coated on one or more surfaces with a layer of acid resisting glass. The invention lies in coating the glass with a layer 4 of fluoroplastic so as to improve the chemical resistance of the glass and to prolong the useful life of the metal vessel. The substrate 2 is preferably of carbon steel or cast iron. Two or more layers of glass may be applied, the layer 5 adjacent to substrate 2 being to promote adhesion between substrate 2 and glass layer 6. Layer 6, 7, 8 are of corrosion resistive glass. <IMAGE>

Description

SPECIFICATION Protective coatings for metals This invention relates to protective coatings for metals, and especially, though not exclusively, to protective coatings forthe interior surfaces of metal vessels.

Some items used in certain types of chemical plant are manufactured out of carbon steel or, occasionally, cast iron, and surfaces of such items which may come into contact with substances likely to react therewith are often coated with chemical resisting glass to give corrosion resistance. Such a fabrication will hereinafter be referred to as glassed steel.

Glassed steel possesses the advantages of excellent corrosion resistance in most acid media and a wide range of solvents, as well as good thermal conductivity. It does however suffer from the disadvantages of relatively poor corrosion resistance to alkali materials and hydrofluoric acid, as well as a certain amount of brittleness when subjected to mechanical shock.

One alternative to the use of glassed steel consists of applying afluoroplastic polymer directlyto the surface of the metal. Such a fabrication will hereinafter be referred to as plastic-steel.

Plastic-steel possesses the advantages of generally good corrosion resistance to a wide range of materials together with a good resistance to mechanical impact It does howeversufferfrom the disadvantagethat such coatings are usualiy micro porous unless very thick coatings are applied. The micro pores tend to lead to thefailure ofthe coating due to corrosion ofthe metal substrate atthe interfacial layer, although the actual coating itself is largely unaffected by the chemicals used. However the solution to the problem of the failure ofthe coating does not usually lie in applying verythickcoatings as such a process tends to be both difficult and expensive.

Hence the use of either glassed steel or plastic-steel does not readily solve the problem of providing steel (or iron) with a relatively cheap corrosion resistant coating.

According to the present invention a protective coatingfora metal substrate comprises a chemically resisting glass or glass ceramic coating bonded directlyto at least one surface ofthe metal substrate, and a layer of a fluoroplastic polymer on the surface of the glass or glass ceramic coating.Such a multi-layer coating has been found, not only to provide protection against a wide range of chemicals, together with the protection ofthe glass coating itselffrom impact damage but, owing to the presence of the glass beneath the polymer layer, alsoto reducethe possibil- ity of corrosion ofthe underlying metal which could otherwise be caused by certain materials penetrating micropores in the polymer layer, thereby significantly increasing the useful life of the metal.

Preferably the glass or glass ceramic coating itself comprises two or more layers, the layer adjacent the metal substrate consisting of a ground-coat of glass whose purpose isto promote adhesion between the said metal substrate and the remaining, corrosion resistive, layer or layers of glass or glass ceramic. It is desirable that the thermal conductive properties of the glass or glass ceremic predominate overthose of the fluoroplastic polymer, and also that the polymer is of sufficient thickness to protectthe glass or glass ceramic coating from mechanical impact damage.It is therefore preferable that the glass or glass ceramic coating have a total thickness of between 0.5 and 2.0m m, and that the fluoroplastic polymer (such as PTFE, PFA or FEP) have a thickness of between 0.025 and 0.5mm with the metal substrate (typically carbon steel or cast iron) having a thickness in the range 5to 35mm.

One embodiment of the invention will now be described, as an example only, with reference to the accompanying drawing (notto scale) in which: Figure 1 represents a cross-section through a metal substrate having a multi-layer protecting coating in accordance with the invention, and, Figure 2 represents a detailed cross-section through a substrate having a modified protective coating.

Referring now to Figure 1 a multi-layer protective coating 1 comprises a chemically resistive glass coating 3, having a total thickness of approximately 1 .Omm, bonded directlyto a carbon steel substrate 2, having athicknessofapproximately 15mm.Afluoro- plastic polymer4, such as FEP, is applied to the other surface of the said glass 3 in a layer approximately 0.075mm in thickness.

Thecompletedcoating retainsthesurfaceprop- erties ofthe polymer4, namely generally good corrosion resistance to a wide range of materials, and additionally protection ofthe glass from mechanical impact damage. The polymer layer4 has a thickness chosen so asto allowthethermal conductive prop erties ofthe glass to predominate buf not so thin as to easily allow significant penetration of corrosive material through the micropores. It will of course be appreciated that even if penetration of the micropores does occur it is unlikely that corrosion ofthe metal will result as the glass coating preventsthe corrosive material from coming into contactwiththe metal substrate.

Referring now to Figure 2 a corrosion resistive coating of glass 3 is sandwiched between a FEP layer 4 and a carbon steel substrate 2 as described in Figure 1 above. The glass coating 3 comprises, in this case, four layers 5,6,7,8 of glass, the layer 5 adjacentthe steel substrate 2 being composed of a ground-coat whose purpose is to promote adhesion between the said substrate 2 and the glass layer 6, with the remaining glass layers 6,7,8 being composed of corrosion resistive glass.

The invention is particularly applicable to the protection of the internal surfaces of metal vessels, for example as employed in the, chemical industry and it will be understood that the invention includes within its scope metal vessels provided internally with a protective multi-layer coating as above described.

1. A protective coating for a metal substrate comprising a chemically resisting glass or glass ceramic coating bonded directly to at least one surface of the metal substrate, and a layer of a fluoroplastic polymer on the surface ofthe glass of glass ceramic coating.

2. A protective coating according to Claim 1

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Protective coatings for metals This invention relates to protective coatings for metals, and especially, though not exclusively, to protective coatings forthe interior surfaces of metal vessels. Some items used in certain types of chemical plant are manufactured out of carbon steel or, occasionally, cast iron, and surfaces of such items which may come into contact with substances likely to react therewith are often coated with chemical resisting glass to give corrosion resistance. Such a fabrication will hereinafter be referred to as glassed steel. Glassed steel possesses the advantages of excellent corrosion resistance in most acid media and a wide range of solvents, as well as good thermal conductivity. It does however suffer from the disadvantages of relatively poor corrosion resistance to alkali materials and hydrofluoric acid, as well as a certain amount of brittleness when subjected to mechanical shock. One alternative to the use of glassed steel consists of applying afluoroplastic polymer directlyto the surface of the metal. Such a fabrication will hereinafter be referred to as plastic-steel. Plastic-steel possesses the advantages of generally good corrosion resistance to a wide range of materials together with a good resistance to mechanical impact It does howeversufferfrom the disadvantagethat such coatings are usualiy micro porous unless very thick coatings are applied. The micro pores tend to lead to thefailure ofthe coating due to corrosion ofthe metal substrate atthe interfacial layer, although the actual coating itself is largely unaffected by the chemicals used. However the solution to the problem of the failure ofthe coating does not usually lie in applying verythickcoatings as such a process tends to be both difficult and expensive. Hence the use of either glassed steel or plastic-steel does not readily solve the problem of providing steel (or iron) with a relatively cheap corrosion resistant coating. According to the present invention a protective coatingfora metal substrate comprises a chemically resisting glass or glass ceramic coating bonded directlyto at least one surface ofthe metal substrate, and a layer of a fluoroplastic polymer on the surface of the glass or glass ceramic coating.Such a multi-layer coating has been found, not only to provide protection against a wide range of chemicals, together with the protection ofthe glass coating itselffrom impact damage but, owing to the presence of the glass beneath the polymer layer, alsoto reducethe possibil- ity of corrosion ofthe underlying metal which could otherwise be caused by certain materials penetrating micropores in the polymer layer, thereby significantly increasing the useful life of the metal. Preferably the glass or glass ceramic coating itself comprises two or more layers, the layer adjacent the metal substrate consisting of a ground-coat of glass whose purpose isto promote adhesion between the said metal substrate and the remaining, corrosion resistive, layer or layers of glass or glass ceramic. It is desirable that the thermal conductive properties of the glass or glass ceremic predominate overthose of the fluoroplastic polymer, and also that the polymer is of sufficient thickness to protectthe glass or glass ceramic coating from mechanical impact damage.It is therefore preferable that the glass or glass ceramic coating have a total thickness of between 0.5 and 2.0m m, and that the fluoroplastic polymer (such as PTFE, PFA or FEP) have a thickness of between 0.025 and 0.5mm with the metal substrate (typically carbon steel or cast iron) having a thickness in the range 5to 35mm. One embodiment of the invention will now be described, as an example only, with reference to the accompanying drawing (notto scale) in which: Figure 1 represents a cross-section through a metal substrate having a multi-layer protecting coating in accordance with the invention, and, Figure 2 represents a detailed cross-section through a substrate having a modified protective coating. Referring now to Figure 1 a multi-layer protective coating 1 comprises a chemically resistive glass coating 3, having a total thickness of approximately 1 .Omm, bonded directlyto a carbon steel substrate 2, having athicknessofapproximately 15mm.Afluoro- plastic polymer4, such as FEP, is applied to the other surface of the said glass 3 in a layer approximately 0.075mm in thickness. Thecompletedcoating retainsthesurfaceprop- erties ofthe polymer4, namely generally good corrosion resistance to a wide range of materials, and additionally protection ofthe glass from mechanical impact damage. The polymer layer4 has a thickness chosen so asto allowthethermal conductive prop erties ofthe glass to predominate buf not so thin as to easily allow significant penetration of corrosive material through the micropores. It will of course be appreciated that even if penetration of the micropores does occur it is unlikely that corrosion ofthe metal will result as the glass coating preventsthe corrosive material from coming into contactwiththe metal substrate. Referring now to Figure 2 a corrosion resistive coating of glass 3 is sandwiched between a FEP layer 4 and a carbon steel substrate 2 as described in Figure 1 above. The glass coating 3 comprises, in this case, four layers 5,6,7,8 of glass, the layer 5 adjacentthe steel substrate 2 being composed of a ground-coat whose purpose is to promote adhesion between the said substrate 2 and the glass layer 6, with the remaining glass layers 6,7,8 being composed of corrosion resistive glass. The invention is particularly applicable to the protection of the internal surfaces of metal vessels, for example as employed in the, chemical industry and it will be understood that the invention includes within its scope metal vessels provided internally with a protective multi-layer coating as above described. CLAIMS

2. A protective coating according to Claim 1 wherein the glass or glass ceramic coating itself comprises two or more layers.

3. Aprotectivecoating according to Claim 2 wherein the layer adjacent the metal substrate consists of a ground coat of a glass which is less corrosion resistantthan the other layer or layers but serves to bond the latter two the substrate.

4. A protective coating according to any preceding Claim wherein the glass or glass ceramic coating has a total thickness of between 0.5mm and 2.0mm and the fluoroplastic polymer layer has athickness of 0.025mm to 0.05mm.

5. A protective coating according to any preceding Claim wherein thefluoroplastic polymer layer consists of PTFE, PFA or FEP.

6. A metal vessel having a protective coating according to any preceding Claim on itsinnersurface.

7. A metal vessel according to Claim 6 consisting of carbon steel or cast iron.

8. A metal vessel according to Claim 7 wherein the wall ofthe vessel has a thickness of between 5mm and 35mm.

9. A metal vessel having a protective coating substantiallyasshown in andashereinbeforede- scribed with reference to Figure 1 or Figure 2 ofthe accompanying drawing.