Press roll with ceramic coating for difficult corrosion conditions and method for manufacture of the roll

Abstract: The present invention concerns a press roll, in particular a centre roll, for a paper/board machine, a method for manufacture of the roll, and a composition of the coating of the roll for difficult corrosion conditions. Onto the frame part (1) of the roll, an adhesion/corrosion-protection layer (2) of a thickness of 150...300 µm has been applied out of a nickel-chromium alloy by means of the high-velocity flame spraying technique (HVOF), onto the adhesion layer (2) a tight-ceramic layer (3) of a thickness of 50...150 µm has been applied by means of the high-velocity flame spraying technique, which layer (3) consists of 50...100 % of Al 2 O 3 and of 0... 50 % of TiO 2 , or, alternatively, of 0...100 % of Al 2 TiO 5 , and onto the tight ceramic layer (3) a porous ceramic layer (4) has been applied.

Description: The present invention concerns a press roll for a paper/board machine, in particular a centre roll, a method for manufacture of the roll, and a composition of the roll coating for difficult corrosion conditions. In the press section of a paper and board machine a roll is used which was, earlier, commonly made of granite. In spite of the excellent surface properties of granite rolls, new rolls, mainly with ceramic coating, have mostly been gradually substituted for the granite rolls. With the prior-art rolls with metal, metal-ceramic and ceramic coatings, however, corrosion of various degrees has occurred in particular in more severe conditions, which corrosion causes corroding of the roll, delamination of the coating, and wear of the roll so that it becomes unusable. The rolls are coated by means of thermal spraying, in which case the coating unavoidably remains porous, and the roll face susceptible of corrosion. In view of corrosion, the process conditions in paper industry have become more and more severe, because, out of reasons of protection of the environment, process water is recirculated to an ever greater extent, and the rolls also must operate under ever more corroding conditions. This is why it is necessary to renew the coatings on rolls and/or to replace the rolls quite frequently, which, of course, causes high expenses. Attempts have been made to improve the resistance of rolls to corrosion by means of a number of methods. In the FI Patent Application No. 853544, a method for coating of a roll is described, in which a metallic core of a roll has been coated fully or partly with a metal, such as stainless steel, in which the proportion of chromium is 9...35 %, and it has been believed that chromium expressly increases the resistance of steel to corrosion. In the FI Patent No. 89,950 , the metal core of a roll has been coated with molybdenum- and nickel-based metals, such as nickel-chromium alloys, nickel-chromium-aluminum alloys, etc. in order to improve the resistance of the roll to corrosion. This intermediate layer is applied onto the roll by means of flame spraying or plasma spraying. The thickness of the intermediate layer is 100...500 µm. Between this porous metal layer and the core of the roll, it is further possible, by means of flame spraying or plasma spraying, to apply a tight metal film that inhibits corrosion and that has a thickness of about 100...500 µm. Metals suitable for this are nickel, aluminum alloy, copper, stainless steel, etc., and the particles that are used for the spray coating material are very small. In the FI Patent No. 86,566 , a roll coating is described in which, on the roll core, there is a corrosion-protection layer, whose thickness is at least 0.5 mm. The tightness of the corrosion-protection layer is higher than 96 % and, if necessary, the corrosion-protection layer may have been sealed by means of laser, induction, plasma, flame, or electron-jet melting, or it may also have been produced by means of thermal spraying. The corrosion-protection layer consists of stainless steel whose chromium content is 10...29 %, and it is placed between the core mantle of the roll and the ceramic outer layer. In the FI Patent No. 82,094 , a coating for a Yankee cylinder in a paper machine is described, whose resistance to corrosion permits manufacture of special papers in acid solutions, in which the pH can be in the range 3...5. The coating has been made of a mixture of a metal powder and a carbide or nitride. In the FI Patent No. 84,506 , a press roll for a paper machine is described, in which the roll core has been coated with an intermediate layer which consists of a composite compound made of a ceramic and a metal, so that the ratio of the components in the composite compound is different in different parts of the intermediate layer in the direction of the radius of the roll. Then the intermediate layer has been coated with a ceramic material. In the EP Patent Application 0,657,237, coating of a roll with tungsten carbide and chromium carbide by means of thermal spraying is described. The function of this coating is also, among other things, to improve the resistance to wear of the carbide coatings on rolls. In the European Patent Application 0,481,321 , a press roll is described in which an intermediate layer made of a molybdenum-based or nickel-based alloy has been applied onto the metal core, for example, by means of plasma spraying, and onto this layer a ceramic layer has been applied by means of the plasma spraying method, which layer consists of metal oxides or of mixtures of same. Finally, the roll is coated with an organic polymer to fill the pores in the porous ceramic coating. In the US Patent 5,272,821, a heated roll is described which comprises a first metallic coating applied to the roll, a second high porosity ceramic coating applied to said first metallic coating and a third low porosity ceramic coating applied to said porous ceramic coating. In the EP Patent Application 0,369,968, various surface material compositions for externally heatable rolls are described. In the prior-art solutions and in the coatings that are used currently on the centre roll in a paper machine, a problem is the softness of the polymer coating or the porosity of the ceramic coating, in which case the medium that produces corrosion of the roll frame, such as moisture and other corroding materials, can penetrate through the small pores through the coating to the roll frame. In this connection, corrosion arises, which deteriorates, among other things, the adhesion of a ceramic coating to the roll and promotes damaging of the metal core of the roll. In particularly difficult corrosive environments, such as rolls that operate under highly acid or alkaline conditions, very rapid corroding has been noticed in spite of a metallic corrosion-protection layer that has been applied to underneath a ceramic coating by means of plasma spraying or high-velocity flame spraying (HVOF) and whose thickness is about 200...300 µm. In such a case, the coating layer of the roll often falls apart partially, i.e. delamination takes place, and the roll is unusable. The object of the present invention is a novel press roll with a ceramic coating for difficult corrosion conditions, a method for manufacture of the roll, and a composition of the coating for the roll. The press roll in accordance with the invention, the method for manufacture of the press roll, and the composition of coating for the press roll in accordance with the invention are characterized in what is stated in the patent claims. By means of the method in accordance with the invention, it is possible to manufacture new rolls and also to coat old rolls from which the earlier coating has first been removed. In order to achieve the objectives of the invention and to solve the problems occurring in the prior art, the procedure is as follows: Onto the mantle 1 of the roll frame, a metallic adhesion/corrosion-protection layer 2 of a thickness of 150...300 µm is applied by means of the high-velocity flame spraying method (HVOF), and onto said layer a very tight ceramic coating layer 3 of a thickness of 50...150 µm is applied by means of the high-velocity flame spraying technique (HVOF) and by using a very finely divided coating material whose melting temperature is, owing to the mixture ratio and to the fine particle size, clearly lower than with an outer coating layer proper produced by means of plasma spraying, and as the last layer a ceramic porous outer coating 4 is applied. In this way, a press roll is obtained that is durable and operates well even under difficult corrosion conditions. When the HVOF method is employed, the high particle velocity (about 800 metres per second) provides the coating with an exceptionally good adhesion and with a very tight structure resistant to corrosion. The coating in accordance with the invention is very well suitable for difficult corrosion conditions. In the method in accordance with the invention, onto the frame part 1 of the roll, an adhesion/corrosion-protection layer 2 is applied by means of the high-velocity flame spraying method (HVOF), which layer consists of a nickel-chromium alloy and has a thickness of 150...300 µm, preferably 180...220 µm, then, onto said layer, a tight-ceramic layer 3 is applied, whose thickness is 50...150 µm, preferably 80...120 µm. The tight-ceramic layer 3 consists of 50...100 % of aluminum oxide (Al 2 O 3 ) and of 0...50 % of titanium dioxide (TiO 2 ), preferably 55...65 % Al 2 O 3 and 35...45 % TiO 2 . The tight-ceramic layer 3 can also be prepared exclusively out of aluminum titanium (Al 2 TiO 5 ), or Al 2 TiO 5 can be added to among Al 2 O 3 and TiO 2 as an amount of 0...100 %. Onto the tight ceramic layer a porous ceramic layer 4 is applied by means of conventional methods out of any desired ceramic grade. One solution in accordance with the invention is illustrated by means of the accompanying Figure 1. List of illustrations: 1. Solution in accordance with the invention. Fig. 1 is a sectional view of the roll face, wherein an adhesion/corrosion-protection layer 2 of a thickness of 150...300 µm, preferably 180...220 µm, that contains a nickel-chromium alloy has been applied onto the mantle 1 of the roll frame, and onto said layer a tight-ceramic layer 3 of a thickness of 50...150 µm, preferably 80...120 µm, has been applied, which layer 3 contains 50...100 %, preferably 55...65 %, of Al 2 O 3 and 0...50 %, preferably 35...45 %, of TiO 2 , and as the last layer a porous outer ceramic layer 4 has been applied.

Claims: A press roll for a paper/board machine, comprising an adhesion/corrosion-protection layer (2) of a thickness of 150...300 µm that has been applied onto the frame part (1) of the roll, which layer (2) consists of a nickel-chromium alloy, characterized in that the roll comprises a tight-ceramic layer (3) of a thickness of 50...150 µm that has been applied onto the adhesion layer (2), which layer (3) consists of 50...100 % of Al 2 O 3 and of 0...50 % of TiO 2 , and a porous ceramic layer (4) that has been applied onto the tight ceramic layer (3). A press roll as claimed in claim 1, characterized in that the thickness of the adhesion/corrosion-protection layer (2) is 180...220 µm. A press roll as claimed in claim 1 or 2, characterized in that the thickness of the tight-ceramic layer (3) is 80...120 µm. A press roll as claimed in any of the claims 1 to 3, characterized in that the tight-ceramic layer (3) consists of 55...65 % of Al 2 O 3 and of 35...45 % of TiO 2 . A press roll for a paper/board machine, comprising an adhesion/corrosion-protection layer (2) of a thickness of 150...300 µm that has been applied onto the frame part (1) of the roll, which layer (2) consists of a nickel-chromium alloy, characterized in that the roll comprises a tight-ceramic layer (3) of a thickness of 50...150 µm that has been applied onto the adhesion layer (2), which layer (3) contains 0...100 % of Al 2 TiO 5 , and a porous ceramic layer (4) that has been applied onto the tight ceramic layer (3). A method for the manufacture of a press roll for a paper/board machine, in which method an adhesion/corrosion-protection layer (2) of a thickness of 150...300 µm is applied onto the frame part (1) of the roll by means of the high-velocity flame spraying technique (HVOF), which layer (2) contains a nickel-chromium alloy, characterized in that a tight-ceramic layer (3) of a thickness of 50...150 µm is applied onto the adhesion/corrosion-protection layer (2) by means of the high-velocity flame spraying technique, which layer (3) consists of 50...100 % of Al 2 O 3 and 0...50 % of TiO 2 , and a porous ceramic layer (4) is applied onto the tight ceramic layer (3). A method as claimed in claim 6, characterized in that the thickness of the adhesion/corrosion-protection layer (2) is 180...220 µm. A method as claimed in claim 6 or 7, characterized in that the thickness of the tight-ceramic layer (3) is 80...120 µm. A method as claimed in any of the claims 6 to 8, characterized in that the tight-ceramic layer consists of 55...65 % of Al 2 O 3 and of 35...45 % of TiO 2 . A method for the manufacture of a press roll for a paper/board machine, in which method an adhesion/corrosion-protection layer (2) of a thickness of 150...300 µm is applied onto the frame part (1) of the roll by means of the high-velocity flame spraying technique (HVOF), which layer (2) contains a nickel-chromium alloy, characterized in that a tight-ceramic layer (3) of a thickness of 50...150 µm is applied onto the adhesion/corrosion-protection layer (2) by means of the high-velocity flame spraying technique, which layer (3) contains 0...100 % of Al 2 TiO 5 , and a porous ceramic layer (4) is applied onto the tight ceramic layer (3).

Inventors: LEHTONEN PENTTI, NIEMI KARI, WAHLROOS JUHA, LEHTONEN, PENTTI, NIEMI, KARI, WAHLROOS, JUHA
Applicants: METSO PAPER INC, METSO PAPER, INC.
Application year: 1998
Main classification: D21F3
IPC codes: D21G1, F16C13, C23C4, D21F3