Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C4/00—Coating by spraying the coating material in the molten state, e.g. by flame, plasma or electric discharge
  • C23C4/18—After-treatment
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C10/00—Solid state diffusion of only metal elements or silicon into metallic material surfaces
  • C23C10/28—Solid state diffusion of only metal elements or silicon into metallic material surfaces using solids, e.g. powders, pastes

Definitions

• the invention relates to a method of forming a protective metallic coating on a substrate (e.g. for providing corrosion resistance); an initial porous metallic coating is provided on the substrate by spraying and then a heat treatment is carried out to sinter at least part of the initial layer and give rise to diffusion of the initial coating into the substrate.
• a substrate e.g. for providing corrosion resistance
• Corrosion is a problem in many areas of industry and it is believed that the effects of high temperature corrosion have recently become more severe. Examples of such forms of high temperature corrosion are vanadium pentoxide attack in boilers, chloride attack in burning furnaces, in fluidised beds, and sulphate attack and attacks by sulphur dioxide and hydrogen sulphide in chemical processing apparatus. Materials that are subject to corrosion may be protected by various coating techniques such as metal plating, metallizing and coating with metal powders and spraying of metals. However, metal plating and metallizing are complicated and expensive, and coating with metal powders and spraying with metals, whilst more straightforward to carry out, give coatings containing voids and pores and also of variable adhesion to the substrate.
• UK Patent Specification No 1 581 172 describes a method of meeting the above problems by forming a surface layer on a stainless steel product by forming an initial layer of from 10p to 2 mm in thickness on the surface of the stainless steel by applying powders of at least one of Fe, Cr, Ni, Ti, Mo, Nb, Co and alloys containing two or more of these metals on the surface, heating the initial layer at a temperature ranging from 1150 to 1480°C for 0.01 second to 10 minutes by means of high frequency heating at a frequency ranging from 0.1 KHz to 500 KHz so as to sinter at least part of the initial layer and to diffuse part of the metals of the initial layer into the stainless steel product to a depth of at least 1 ⁇ , and to give a surface layer with a porosity of not more than 4%.
• the invention is concerned with application of the method of UK Patent No 1 581 172 to production of metallic coatings, such as Al-bearing ferritic steel coatings, and with certain modifications such as provision of a working step thereby to give rise to a coating of increased smoothness and density.
• the invention provides a method of forming a protective metallic coating on a refractory metal substrate which comprises the steps of
• Coatings so produced have been found to be dense, adherent, and ductile and also to provide good corrosion resistance as evidenced by tests described herein. Also they may have a high quality surface finish which is important from the corrosion resistance aspect.
• the invention is applicable to the protection of a substrate in a variety of forms, for example a tube, metal sheet or strip and a shaped article.
• the substrate- may, for example be an article such as a condenser, a fluidised bed component or a component for chemical process apparatus, or a complex shaped component such as a blade for a gas turbine engine, or may be a material for subsequent fabrication into such an article.
• the substrate may, for example, be made of a steel such as a stainless steel or a mild steel. Low quality substrates such as the latter may therefore be improved by means of the present invention.
• the protective metallic coating may, for example, be a single element coating, e.g. Ti or Zr, or an alloy.
• it is a steel coating and is most preferably an aluminium-bearing ferritic steel coating, which steel may, for example, be a steel containing Fe, Cr and Al with or without additional constituents such as Y and/or Hf.
• a preferred Al-bearing ferritic steel is a steel containing Fe, Cr, Al and Y, for example as marketed under the UK Registered Trade Mark "FECRALLOY", because of its outstanding ability to withstand corrosion in aggressive environments such as those containing S, C, N and halogen.
• the Al-bearing ferritic steel used may contain additional constituents such as Si which may improve the corrosion resistance of the steel and also incidental constituents which may be present as a result of the materials used in manufacture thereof and of the manufacturing process itself.
• Preferred Al-bearing ferritic steels in the invention are those having high Al concentrations, for example up to 8% or up to 10% by weight, and/or those having high Cr concentrations, for example up to 25% by weight.
• Al-bearing ferritic steels of such high Al and/or Cr concentrations are normally difficult to fabricate when in the form of coatings. The invention however does not require fabrication to be carried out and is therefore particularly advantageous in the provision of coatings of such steels on shaped articles.
• Particularly preferred Al-bearing ferritic steels are those containing from 10% to 25% Cr, from 2 to 10% Al, up to 1% Y, up to 0.5% Si and the balance Fe where all proportions quoted are by weight.
• Step (i) may be carried out by methods known in the art for producing metallic coatings by spraying, for example by plasma jet spraying or by gas spraying.
• the spraying is carried out in an inert environment such as an atmosphere of argon in order to minimise oxidation and give a substantially oxide-free initial coating.
• an atmosphere of argon such as an atmosphere of argon
• Step (ii) may be carried out, for example, at a temperature in the range from 950°C to 1300°C such as 1100°C. It may, for example, be carried out by means of high frequency heating which is particularly appropriate when the substrate is in elongated form such as a tube since the latter can readily be heated by causing a high frequency heating coil to traverse the length of such a substrate.
• Other forms of heat treatment that may possibly be suitable are conventional furnace heating and laser heating. The latter may be suitable for heat treating coatings on the insides of tubes which are normally difficult to heat by means of high frequency heating.
• Step (iii) may be caried out by methods known in the art and its general effect is to densify the coating. It is preferably carried out as a warm working step such as warm rolling, for example by working the product of step (ii) before it has completely cooled down, e.g. at a temperature in the range of 600°C to 800°C.
• a warm working step such as warm rolling
• the substrate is elongated such as a tube
• steps (ii) and (iii) sequentially along the length of the substrate e.g. by arranging for a means for heat treating the coated substrate (such.as an H.F. coil) and a means for working the coated substrate (such as rollers) to traverse the coated substrate together at a spaced interval.
• steps (i), (ii) and (iii) may be carried out sequentially, e.g. by arranging a spraying means, such heat treating means and such means for working to traverse the substrate together at spaced intervals from one another.
• “Traverse” is not necessarily to be taken to mean that the substrate is always stationary although usually this will be the case. Thus, “traverse” means that there is relative movement between the substrate and the means for effecting any of steps (i), (ii) and (iii) as appropriate.
• step (iii) is carried out after step (ii). However, it may be possible to carry out step (ii) before step (iii) with a view to improving the surface finish of the coating.
• Further densification of the coating may be effected by cold working operations such as drawing through a die, swaging or reeling.
• the thickness of the coating in the invention may be up to . l2 mm, for example 1 mm, though it may be possible to produce thicker coatings. Also, it may be possible to carry out the invention more than once in order to produce a thicker coating.
• the coated substrate may be subject to further treatment in order to enhance the performance of the coating.
• a coating that contains less Al than intended due to losses thereof in step (i) as discussed above may be alonized in order to increase the Al content.
• Alonizing is a process for diffusing Al into the surface of Fe base or Ni base alloys and is described in "Alonized Heat Exchanger Tubes Give Good High Temperature Service” by McGill and Weinbaum at Corrosion/76, March, 1976, Houston, Texas.
• the final tube was tested by taking a i" thick ring from the tube, slitting the ring and opening up the slit until the ring was almost flattened. It was observed that the coating remained adherent.
• a similar test was carried out on a tube coated according to step (i) only above (i.e. without carrying out steps (i) and (iii). When the slit was opened to a gap of about 2", the coating was observed to spring away from the surface of the substrate.
• the final tube was tested as described in Example 1 and the coating was found to be adherent. Micro-examination of the final tube showed a marked reduction in the porosity of the coating.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Plasma & Fusion (AREA)
• Other Surface Treatments For Metallic Materials (AREA)
• Coating By Spraying Or Casting (AREA)

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Publications (1)

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Cited By (4)

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Citations (5)

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• 1983
  • 1983-06-20 EP EP19830303527 patent/EP0098085A1/de not_active Withdrawn
  • 1983-06-20 GB GB08316768A patent/GB2122651B/en not_active Expired

Patent Citations (5)

Non-Patent Citations (3)

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