Corrosion preventing structure

Abstract: The invention is a corrosion preventing structure for preventing corrosion of metallic parts in which a secondary plating film (3) is formed on the primary plating film (2) comprising a Ni-P type alloy, said secondary plating film being a Ni-P/PTFE composite plating film prepared by dispersing and deposition of polytetrafluoroethylene fine particles (4) in a Ni-P type alloy.

Description: This invention relates to a corrosion reducing structure and a method of protecting a surface from corrosion. The structure and method are particularly, but not exclusively, useful for protecting metallic parts of a vacuum pump used for gas discharging from a semiconductor producing apparatus, especially from a dry etching apparatus. Since an intensively corrosive gas is used in some semiconductor producing apparatuses, it is necessary to use an excellent anticorrosive material for construction of the portions which are exposed to a corrosive gas including, not limitedly, the inside of chambers, valves and pipings of such apparatuses, but even vacuum pumps for gas discharging from the chambers. In general, austenite type stainless steels typified by SUS304 are used as the anticorrosive material of this kind. That is, it is common practice to use an austenite type stainless steel for constructing those areas exposed to a corrosive gas, but a high power aluminium alloy is used for preparing those parts which rotate at a high speed such as rotating elements of vacuum pumps since light weight and high strength are required. In the case of such aluminium alloys, an oxide film is spontaneously formed on the surfaces thereof and it functions as a passive state coating so that the surfaces are endowed with a corrosion resisting property to a certain extent. However, since the corrosion resisting property of aluminium alloys is considerably inferior to that of the passive state coating on stainless steels, the oxide films on the surfaces of aluminium alloy are broken when exposed to a corrosive gas as the exhaust gas to cause corrosion thereon. Therefore, it is necessary to apply a corrosion preventing treatment to the surface of aluminium alloys other than the spontaneously formed oxide film. In view of this, surfaces of aluminium alloys have hitherto been subjected to electroless plating with a Ni-P type alloy or to anodic oxidation treatment (alumite process) as the anticorrosion treatment. The plating treatment with a Ni-P type alloy is carried out by applying the electroless plating method which is distinctive from the usual electroplating. On the whole surface of the parts made of an aluminium alloy a Ni-P type alloy such as Ni-P, Ni-W-P or the like is deposited to form a film of 10 to 25 micrometer thickness by this method. Also, the anodic oxidation treatment should be accompanied with the so called sealing treatment for plugging the micropores of oxide film formed by the usual treatment. However, in the apparatuses which use a chlorine type gas having intensively corrosive action such as Cl 2 , CCl 4 , BCl 3 , and the like such as the recently developed reactive ion etching apparatus (RIE) for aluminium alloys, it is not possible to protect the aluminium alloys with the traditional coating films formed by the treatment of electroless plating with a Ni-P type alloy or alumite treatment bringing about a problem of corrosion of the aluminium alloy. The mechanism of corrosion is as follows. Etching of aluminium alloys is practised by ionisation of a chlorine type gas to form chlorine ions which are crashed against the aluminium film on a silicon substrate. At this time, the reaction product (AlCl 3 ) is generated in a large amount as a vapour and the vapour deposits in the course of discharging on the sites where the temperature is low and the pressure is high, namely the inner surface of exhaust pump. Such a deposited product (AlCl 3 ) sublimates at 178°C under 1 atm and at about 40°C under 0.3 Torr. Therefore, the deposited product (AlCl 3 ) of this kind reacts with atmospheric moisture in the event of pump suspension, leaking or the like to form HCl, namely chlorine ions. Also, chlorine ions may be formed by the reaction with moisture at the time of periodical maintenance or washing elimination. Chlorine ions readily break the passive state films on aluminium alloys and stainless steels inducing corrosion of pit form (pitting corrosion). Once pitting corrosion is induced, the sites act as local cells and corrosion advances acceleratively. Further, in the apparatuses of such a type, since raw material gases having intensive corrosiveness such as Cl 2 , BCl 3 and the like are partly discharged as they are through the exhaust pump, such gases are adsorbed by the inner surface of relevant exhaust pump intact and sometimes generate chlorine ions similarly to the above. Through the above procedures a large amount of chlorine ions are generated. Confronting the existence of such a large amount of chlorine ions, the electroless plating film of a Ni-P type alloy of 20 micrometer thickness and the alumite processed coating film having been hitherto applied as an anticorrosion treatment cannot completely prevent corrosion. That is to say, chlorine ions readily intrude through the micropores (pinholes) existing in the plating films to reach the aluminium substrate where pitting corrosion is induced and, once pitting corrosion is induced in this manner, the local cell function is by far amplified standing on the relevance to the Ni alloy existing in the plating films thereby to intensively advance pitting corrosion so that the corrosion product forces up the plating films to induce exfoliation of the plating films. The electroless plating treatment is preferable as a treatment to form the coating films for corrosion prevention in respect of the capability of promoting growth of the film even in the inside of concave areas and holes with a thickness equal to that on the flat portions. Further, the Ni-P alloy per se is not attacked by chlorine ions. However, with the electroless plating treatment, it is not possible for coating films to completely get rid of pinholes in the coating films and some sites cannot be plated particularly in the concave areas of machining processed surfaces and electric discharge machined surfaces or the areas of nonhomogeneous aluminium texture. With respect to this point, it is postulated that pinholes piercing from the surface of aluminium to the surface of plating film are sometimes generated in the cases in which the object of electroless plating is aluminium. It is difficult for the known plating technology to completely eliminate such piercing pinholes and, when being intact, chlorine ions reach the aluminium substrate by way of such pinholes starting from the plating surface thereby to cause unavoidably corrosion of aluminium. Prior art can be found in JP7080876 which discloses a mould having a composite film layer 3, which is produced through the uniformly dispersed eutectoid of finely divided polytetrafluoroethylene (PTFE) particle in electroless plating plated through ground layer 2 on the surface 11 of the base of mould base material 1 such as aluminum alloy or the like. At the formation of the composite film layer 3, the base material 1 is buffed, cleansed with ethane and degreased and cleaned with weak alkali so as to form the ground layer 2 for zinc film on the surface 11 of the base. Next, PTFE is dispersed in electroless nickel plating solution having hypophosphate as reducer. After that, composite film layer 3 is preferably formed by directly immersing the base material 1, on which the ground layer 2 is formed, in the mixed liquid just mentioned above at normal temperature for 10-24 hours. It is an object of the present invention to provide a corrosion preventing structure which is suitable for metallic parts composed of an aluminium alloy, iron and the like. According to one aspect of the present invention, there is provided a corrosion reducing structure applied to a surface to be protected, the structure comprising a first plating film composed of Ni-P type alloy for application to the surface, characterised in that a second plating film comprising fine particles in a Ni-P type alloy is provided on said first plating film. According to another aspect of the present invention, there is provided a method of protecting a surface against corrosion, the method comprising applying a first plating film composed of Ni-P type alloy to the surface, and characterised by applying a second plating film comprising fine particles in a Ni-P type alloy over said first plating film. The surface may, for example, be an aluminium alloy or an iron type material. In order to attain the above mentioned object, the present invention, in one aspect, is featured by and has adopted a scheme that a primary plating film comprising a Ni-P type alloy has been formed on the surfaces of metallic parts and a secondary plating film of Ni-P type alloy containing dispersed and deposited fine particles has been formed on the primary plating film. Further, a primary plating film comprising a Ni-P type alloy may be formed at least on the portion of aluminium alloy in the turbomolecular pump used for gas discharging from a semiconductor producing apparatus and a secondary plating film of a Ni-P type alloy prepared by dispersing and deposition of fine particles may be formed on this primary plating film. Further, a primary plating film comprising a Ni-P type alloy may be formed on at least aluminium alloy part of a dry pump used for gas discharging from a semiconductor producing apparatus and a secondary plating film of a Ni-P type alloy prepared by dispersing and deposition of fine particles may be formed on this primary plating film. Further, a primary plating film comprising a Ni-P type alloy may be formed at least on an inner surface of a piping used for gas feeding of a semiconductor producing apparatus or used for gas discharging from a semiconductor producing apparatus and a secondary plating film of a Ni-P type alloy prepared by dispersing and deposition of fine particles may be formed on this primary plating film. Further, a primary plating film comprising a Ni-P type alloy may be formed at least on a moving part of a valve used for gas feeding of a semiconductor producing apparatus or used for gas discharging from a semiconductor producing apparatus and a secondary plating film of Ni-P type alloy prepared by dispersing and deposition of fine particles may be formed on this primary plating film. Further, a primary plating film comprising a Ni-P type alloy may be formed at lest on a moving part and a sliding part in a chamber of a semiconductor producing apparatus and a secondary plating film of a Ni-P type alloy prepared by dispersing and deposition of fine particles may be formed on this primary plating film. Further, the metallic part may be composed of a material of aluminium alloy type or of iron type. Further, polytetrafluoroethylene may be used as the fine particles. Further, the fine particles may be about 1 micrometer or less of the particle diameter. Further, the primary and the secondary plating films may be respectively, 8 micrometers or more of the film thickness. Further, the Ni-P type alloy may be compounded with the fine particles in a ratio of 20 vol% or more by volume ratio or 6 wt% or more by weight ratio. The fine particles are of a size such that they fill the holes (micropores) which are present in the primary plating film. In this invention, metallic parts may be firmly protected against intrusion of chlorine ions to the substrate thereof an occurrence of corrosion in a pit form can be prevented or reduced. It is thought that such a phenomenon is due to pinholes having an opening at the surface of the primary plating film being plugged by the fine particles, and that if the pinholes begin to from during growth of the secondary plating film, the pinholes are immediately plugged by the fine particles and growth of the pinholes is disrupted by the fine particles preventing pinholes from straightforwardly piercing toward the metallic parts. For a better understanding of the invention, embodiments will now be described by way of example, with reference to the accompanying drawings in which: Fig. 1A and 1B are explanatory diagrams of the corrosion preventing structure according to this invention; Fig. 2 is a photograph showing a cross section of a product according to this invention; Fig. 3 is a photograph showing another cross section of a product according to this invention; Fig. 4 is a photograph showing another cross section of a product according to this invention; Fig. 5 is a photograph showing a cross section of a prior art product A; Fig. 6 is a photograph showing another cross section of a prior art product A; Fig. 7 is a photograph showing a cross section of a prior art product B; Fig. 8 is a photograph showing another cross section of a prior art product B; Fig. 9 is a photograph showing a cross section of a comparative sample C; Fig. 10 is a photograph showing another cross section of a comparative sample C; Fig. 11 is a photograph showing another cross section of a comparative sample C; Fig. 12 is a photograph showing a cross section of a comparative sample D; Fig. 13 is a photograph showing another cross section of a comparative sample D; Fig. 14 is a photograph showing another cross section of a comparative sample D. A detailed description s given below on the corrosion preventing structure according to this invention referring to Fig. 1 through Fig. 14. This corrosion preventing structure has, as shown in Fig. 1, a primary plating film 2 comprising a Ni-P type alloy on the surface of metallic part 1 and is further provided with a Ni-P/PTFE composite plating film as the secondary plating film 3 on the primary plating film 2. The Ni-P/PTFE composite plating film is formed with not solely a Ni-P alloy but by dispersing and deposition of polytetrafluoroethylene in the form of fine particles (hereinafter termed as "PTFE fine particles") in this Ni-P type alloy. Such stratified two layers of plating films 2, 3 can be formed through the following procedures. (1) The first stage of treatment The known treatment of electroless plating with a Ni-P alloy is applied to the surfaces of metallic parts 1 such as rotary wings, fixed wings and other well known as aluminium alloy parts of turbomolecular pumps thereby to form the first layer of a Ni-P alloy (the primary plating film 2). That is to say, after applying the prescribed pretreatment to the metallic part 1, this metallic part 1 is dipped in a plating bath having a specified bath composition thereby for forming a Ni-P alloy plating film on the surface of metallic part 1. The concentration of P in the Ni-P alloy plating film should be about 8 wt% and the target value of thickness should be at lest 10 micrometer or more. In consideration of the tolerance and scattering, the film thickness may also be set at 8 micrometers or more. The tendency of the above mentioned pinhole plugging becomes more prominent as the thickness of deposited Ni-P alloy plating film is increased. That is, the number of pinholes having the opening at the plated surface is decreased by forming a thick plating film and the amount of chlorine ions intruded from the surface of plating to the substrate is reduced to allow improvement of the character of corrosion resistance. Accordingly, by taking also economy into consideration, it is preferable to provide the Ni-P alloy plating film formed as the primary plating film 2 with a thickness of about 20 micrometers. (2) The second stage of treatment After formation of the first layer (the primary plating film 2) in the above manner, the Ni-P/PTFE composite plating film is further formed as the second layer (the secondary plating film 3) on this first layer. The Ni-P/PTFE composite plating film should have 10 micrometers of thickness as the target value. The film thickness may also be 8 micrometers or more similarly in this case from the same consideration of the tolerance and scattering as above. The Ni-P/PTFE composite plating film is formed by using a bath prepared by mixing fine powder of PTFE having about 1 micrometer or less of the particle diameter and a surface active agent in a bath almost the same as that for Ni-P alloy plating treatment and by depositing the plating from the bath in a state of being vigorously stirred. That is, the PTFE fine particles are simultaneously dispersed and deposited in the Ni-P alloy plating film. The formulation should be adjusted to regulate the PTFE content in the Ni-P alloy plating film to the level of 20 vol% or above but 40 vol% or lower by volume ratio or 6 wt% or above but 12 wt% or lower by weight ratio. That is to say, the corrosion preventing structure of this example is constructed by forming the primary plating film 2 comprising a Ni-P type alloy on the metallic part 1 and further forming thereon the secondary plating film 3 prepared by dispersing and deposition of PTFE fine particles 4 in the Ni-P type alloy. By this structure, as considered from the viewpoints of following (I) and (II), the structure of metallic part can be securely protected from intrusion and reaching of chlorine ions, and pitting corrosion is not brought about proving that it is a favourable corrosion preventing structure for metallic parts. (I) As shown in Figure 1A, even if pinholes H having an opening at the surface of primary plating film 2 exist at the beginning of formation of the secondary plating film, it is conjectured that such pinholes H are plugged by the PTFE fine particles and intrusion of chlorine ions into such pinholes H is prevented. Also, growing up of pinholes ceases at this point by being plugged in this manner. That is, the fine particles of PTFE 4 impede growth of the pinholes H and prevent pinholes from piercing through the primary plating film 2 to the base material. (II) As shown in Figure 1B, even if appearance of pinholes begins during growth of the secondary plating film 3, such pinholes are immediately plugged by the PTFE fine particles and growth of the pinholes is disrupted by the PTFE fine particles so that pinholes which pierce through the base material do not appear and consequently the intruding sites of chlorine ions scatter over a broad range and localised intensive pitting corrosion only occurs with difficulty. In this corrosion preventing structure, the surface of secondary plating film 3 is covered by PTFE of the amount of 20 to 40 vol% by volume ratio after the growth thereof. Accordingly, the surface of secondary plating film exhibits good water repellency so that substances such as AlCl 3 , Cl gases, chlorine ions and the like come to be adsorbed by the surface of secondary plating film 3 only with difficulty and reaching and intrusion of chlorine ions to the substrate of metallic part can be prevented also in this respect. The effect of preventing intrusion of chlorine ions mentioned above cannot be sufficiently exhibited if the content of PTFE in the secondary plating film 3 is low. For instance, the protection against intrusion of chlorine ions is lowered in the event of the PTFE content of about 5 to 15 vol% by volume ratio (1.5 to 5 wt% by weight ratio), though protection against intrusion of chlorine ions does not become inefficacious. Accordingly, the PTFE content is required to be 20 vol% or more but 40 vol% or less by volume ratio, or 6 wt% or more but 12 wt% or less by weight ratio as mentioned above. The above function and effect have been confirmed by an experiment. An explanation will now be given of this experiment. In this experiment, rotary wings (No. 2000 type high power aluminium alloy) of a turbomolecular pump were used as the metallic part 1 and they were subjected to any of the treatments (i) to (v) mentioned below for preparing a product according to the present invention, prior art products A, B and comparative samples C, D. These test samples are laid in a desiccator the bottom cavity of which was charged with hydrochloric acid diluted with water. In this manner, the above test samples were exposed to vapour of hydrochloric acid. The concentration of hydrochloric acid in this case was at lowest 18 ppm and the time for exposure was 148 hours. Samples were produced as follows: (i) electroless plating wtih a Ni-P alloy (thickness 10 micrometers), then electroless plating treatment with a Ni-P/PTFE (thickness 10 micrometers, PTFE content 10 wt%): the product according to the present invention. (ii) electroless plating treatment with a Ni-P alloy: prior art product A (iii) alumite processing (thickness 8 micrometers): prior art product B. (iv) electroless plating treatment with a Ni-P alloy (thickness 50 micrometers): comparative sample C. (v) electroless plating with a Ni-P alloy (thickness 10 micrometers), then electroless plating treatment with a Ni-P/PTFE (thickness 10 micrometers, PTFE content 5 wt%): comparative sample D. A portion of each test sample, specifically the tip of rotary wing, was cut after the experiment and the cross section was observed and photographed. In the case of the product according to the present invention, Figure 2 to Figure 4 respectively show that intrusion of chlorine ions has been completely prevented and no pitting corrosion has been brought about. In contrast, intensive pitting corrosion is observed in whole surfaces of prior art product A shown in Figure 5 and Figure 6 as well as of the prior art product B shown in Figure 7 and Figure 8. As seen in Figure 9 to Figure 11, in the case of Ni-P alloy plating in which the film thickness is 50 micrometer but not 20 micrometer like the case of comparative sample C, the number of pitting corrosion slightly decreases owing to plugging of the pinholes but pitting corrosion invariably occurs even in this case and complete prevention of pitting corrosion is impossible. further, even in the case where 2-layer plating is applied, intrusion of chlorine ions cannot be prevented as shown in Figure 12 to Figure 14 in the event of little amount of PTFE like in the case of comparative sample D to allow occurrence of pitting corrosion. That is, intrusion of chlorine ions cannot be prevented and occurrence of pitting corrosion remains unavoidable only by increasing the thickness of Ni-P alloy plating film from 20 micrometers to 50 micrometers or only by stratifying the Ni-P alloy plating film in two layers without compounding PTFE. Also, it can be conjectured that even if a Ni-P/PTFE plating (PTFE content 10 wt%) is applied, use thereof as a sole layer is ineffective to avoid occurrence of pitting corrosion. The corrosion preventing structure of this invention is favourable as a measure for corrosion prevention of aluminium alloys by may further be applied to, for example, a corrosion preventing structure of metallic parts constituted with an iron type material. The corrosion preventing structure of this example can be applied to, for example, the following metallic parts. As a matter of course, it can be applied to other metallic parts. (a) aluminium alloy parts of a turbomolecular pump useful for gas discharging from semiconductor producing apparatuses; (b) aluminium alloy parts, or aluminium alloy parts with parts of another metallic material of dry pumps used for gas discharging from similar apparatuses; (c) inner surface, or both inner and other surfaces, of pipings used for gas feeding of similar apparatuses or for gas discharging from similar apparatuses; (d) movable parts, at least, of valves for gas discharging from similar apparatuses; (e) movable parts and sliding parts, at least, in the chamber of similar apparatuses. Although PTFE is used as the fine particles in this Example, there is no need of restriction thereto and any of fine particles can be applied as a replacement of PTFE provided that it can exhibit functions and effects similar to those of PTFE, that is, plugging of pinholes, inhibitio of growth thereof and the like. As the fine particles which exhibit performance and effect equivalent to those of PTFE, fluorinated graphite, ceramics and the like may be referred to, for examples. The corrosion preventing structure is set up by forming a primary plating film composed of a Ni-P type alloy on the surface of metallic parts and further forming a secondary plating film prepared by dispersing and deposition of fine particles in a Ni-P type alloy thereon. Therefore, from the viewpoints of under mentioned (A) and (B), it is possible securely to prevent intrusion and reaching of chlorine ions to the substrate of metallic parts so that pitting corrosion never occurs proving that is a favourable corrosion preventing structure for metallic parts which will extend the life of metallic parts. (A) It is postulated that, even if the primary plating film includes pinholes having the opening at the surface thereof at beginning of formation of the secondary plating film, such pinholes are plugged by the fine particles and intrusion of chlorine ions in the pinholes is prevented. Further, growth of the pinhole terminates by being plugged. That is, the fine particles inhibit growth of pinholes and pinholes piercing straightforwardly to the base material (metallic parts) disappear. (B) Even if formation of pinholes begins in the course of growing up of the secondary plating film, such pinholes are immediately plugged by the fine particles and growth of pinholes is disrupted so that pinholes straightforwardly piercing toward the base material (metallic parts) disappear consequently to scatter the intruding sites of chlorine ions over a broad range and localised intensive pitting corrosion becomes to be only difficulty brought about.

Claims: A corrosion reducing structure applied to a surface (1) to be protected, the structure comprising a first plating film (2) composed of Ni-P type alloy for application to the surface (1), characterised in that a second plating film (3) comprising fine particles in a Ni-P type alloy is provided on said first plating film (2). A method of protecting a surface (1) against corrosion, the method comprising applying a first plating film (2) composed of Ni-P type alloy to the surface (1), and characterised by applying a second plating film (3) comprising fine particles in a Ni-P type alloy over said first plating film (2). Use of a composition of fine-particles in a Ni-P type alloy for improving the corrosion resistance of a Ni-P type alloy coating. The structure, method or use as claimed in claims 1, 2 or 3, wherein the fine particles are of polytetrafluoroethylene (PTFE). The structure, method or use as claimed in claims 1, 2, 3 or 4, wherein the particle diameter of the fine particles is substantially 1 micrometer or less. The structure or method as claimed in claim 1 or 2, wherein the first and second films have, respectively, a thickness of substantially 8 micrometers or more, or the use as claimed in claim 3, wherein the Ni-P type alloy coating and the composition of fine particles in Ni-P alloy, respectively, have a thickness of substantially 8 micrometers or more. The structure, method or use as claimed in any preceding claim, wherein the content of the fine particles in the Ni-P type alloy is 20% or more by volume ratio or 6% or more by weight ratio. The structure, method or use as claimed in claim 7, wherein the content of the fine particles in the Ni-P type alloy is 40% or less by volume or 12% or less by weight ratio. The structure, method or use as claimed in claim 1, 2, 3 or claims 5 to 8, wherein the fine particles are of fluorinated graphite or ceramics. Use of a structure as claimed in claim 1 for reducing corrosion of: an aluminium alloy part of a turbomolecular pump used for gas discharging from a semiconductor producing apparatus; an aluminium alloy part of a dry pump used for gas discharging from a semiconductor producing apparatus; an inner surface of a pipe used for gas feeding of a semiconductor producing apparatus; a movable part of a valve used for gas feeding of a semiconductor producing apparatus or used for gas discharging from a semiconductor producing apparatus; and/or a movable part and a sliding part in a chamber of a semiconductor producing apparatus.

Inventors: KINOSHITA YUICHI, SUDO TAKASHI, YAMANE SHUJI, KINOSHITA, YUICHI, SUDO, TAKASHI, YAMANE, SHUJI
Applicants: SEIKO SEIKI KK, SEIKO SEIKI KABUSHIKI KAISHA
Application year: 1996
Main classification: C23C28
IPC codes: C23F15, H01L21, C23F4, C23C28, C23C18, B05D7