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
  • 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
  • C23C10/30—Solid state diffusion of only metal elements or silicon into metallic material surfaces using solids, e.g. powders, pastes using a layer of powder or paste on the surface
  • C23C10/32—Chromising
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
  • Y10T428/12771—Transition metal-base component
  • Y10T428/12806—Refractory [Group IVB, VB, or VIB] metal-base component
  • Y10T428/12826—Group VIB metal-base component
  • Y10T428/12847—Cr-base component
  • Y10T428/12854—Next to Co-, Fe-, or Ni-base component

Definitions

• a porous adherent chromium-containing coating is applied to the surface of a ferrous metal substrate.
• the coated substrate is then heated in contact with a gaseous stream containing a hydrogen halide and also hydrogen and/or in inert gas to a temperature such that the hydrogen halide reacts with the surface of the coated substrate said temperature being below that at which any appreciable amounts of the resultant metal halides would be removed therefrom by evaporation.
• the flow of the gas is then stopped and the coated substrate heated to a temperature of a least 750 C. for a period sufficient to alloy the chromium in the initial coating with the substrate and form an alloy coating of the desired thickness.
• the present invention relates to the chromising of ferrous metal substrates.
• a chromous halide vapor is presented to the surface of the substrate at a high temperature.
• the chromous halide interacts with the iron of the substrate to yield chromium metal and ferrous halide.
• the deposited chromium then diffuses into the surface of the substrate forming a chromium/iron alloy surface which is corrosion resistant.
• the article to be chromised is introduced into a furnace which is then purged of air by the passage therethrough of a non-oxidising gas and the temperature is raised to the operating temperature which is in excess of 840 C. and usually of the order of 1000" C.
• a hydrogen halide is incorporated into the gas stream passing through the furnace.
• the gases containing hydrogen halide are then passed over a source of chromium, which may be sited either in a separate furnace or in the furnace containing the workpiece, to form the chromous halide required for chromising.
• Another method for achieving chromising is to pack the workpiece in a powder mixture containing an ammonium halide and powdered chromium or an alloy thereof and to heat the loaded furnace to the operating temperature of at least 900 C.
• the ammonium halide decomposes to yield a hydrogen halide which reacts with the chromium present to form the Patented June 29, 1971 chromous halide required. It is usual to carry out this method of chromising in a sealed furnace and not to pass a gas stream therethrough.
• the ferrous halide which is liberated from the surface of the workpiece during chromising, vapourises and forms part of the furnace atmosphere.
• a non-oxidising gas is passed continuously through the furnace substantially all of this ferrous halide is removed from the furnace.
• the ferrous halide interacts with the chromium source to yield chromous halide which is used in the chromising process.
• the source of chromium gradually accumulates iron therein.
• the present invention provides a process for the chromising of ferrous metal substrates which comprises:
• the source of chromium is already bonded to the substrate surface, the iron deposited in the chromium is not wasted, as in the pack process and those processes in which ferrous chloride is removed from the furnace, but forms an integral part of the final alloy layer on the substrate surface.
• the chemical ferrising of the original chromium layer in the process of this invention aids the thermal ferrising which is also taking place in the chromising operation, since it leads to more rapid formation of the chromium/iron alloy layer. If our belief as to the mechanism of the process is true, it is this factor which enables the process to operate at a lower temperature than the known processes.
• adherent is used herein to mean that the coating must be sufficiently adherent to the surface of the ferrous metal substrate to enable it to be handled during transfer from the coating operation to the furnace, or, in the case of steel strip, to permit the strip to be coiled without the coating becoming detached.
• the chromium-containing coating may be deposited upon the surface of the substrate by any well known methods. Such methods include electrolytic deposition of chromium from conventional chromium plating solutions, plasma or flame spraying of a chromium containing powder or wire and the compaction by a rolling technique of a chromium containing powder previously distributed over the surface of the ferrous metal.
• the amount of chromium which is initially applied in the coating of the substrates depends upon the final use to which the treated substrate is to be put and the properties desired for such an end use. Whilst the process of the invention may be used to incorporate very small amounts of chromium in the alloy coating of the substrate, it is of especial application in the formation of corrosion-resistant surface alloys which contain at least 13% chromium. Where mild steel is being chromised to produce a corrosion resistant coating, it is usual to provide a chromium/iron alloy layer on the surface of the mild steel which is 0.002 to 0.003 inch thick. In applications Where the mild steel is to be drawn or formed after chromising, it is desirable that the coating should not have too high a chromium content.
• an initial coating must be applied to the substrate at a rate of from 11 to 17 grams of chromium per sq. ft. of the surface of the substrate.
• the initial coating of the substrate must be porous in order to permit difiusion of the chromous and ferrous halide vapours therethrough.
• the porosity depends to a large extent upon the thickness of the coating and the method by which it is applied to the substrate. Thus, if the chromium is electrolytically deposited upon the substrate the maximum thickness which may be deposited without serious loss of porosity is of the order of 0.001 inch.
• the coating may be up to 0.003 inch thick.
• the method used to achieve this coating is determined by whether or not the particular method produces a coating which is porous enough to facilitate dilfusion of the chromous and ferrous halides therethrough.
• the application of a coating which is from 0.0001 to 0.001 inch thick provides a satisfactory result with a variety of coating methods.
• the coated substrate is then subjected to a first heating stage (stage 2) in a furnace having a non-oxidising atmosphere containing a hydrogen halide.
• a first heating stage stage 2
• a furnace having a non-oxidising atmosphere containing a hydrogen halide In order to obtain the non-oxidising atmosphere, it is preferred to purge the furnace of oxygen before commencing the heating operation by passing therethrough a non-oxidising gas.
• the non-oxidising gas may be hydrogen, argon or other inert gases.
• the hydrogen halide may be introduced into the furnace atmosphere either directly as the anhydrous gaseous acid or by passing the non-oxidising component of the atmosphere through an aqueous solution of the acid and subsequently drying the gas stream before it enters the furnace. Alternatively the halogen may be introduced with hydrogen into the furnace.
• the hydrogen halide may on the other hand be generated by a compound which dissociates on heating to form a hydrogen halide.
• a compound which dissociates on heating to form a hydrogen halide for example, there may be included in the furnace charge an ammonium or other halide which decomposes on heating to give the hydrogen halide.
• the hydrogen halide employed be hydrogen chloride or hydrogen bromide.
• the temperature at which the first heating stage is carried out is that at which reaction between hydrogen halide and the coating and/ or exposed iron on the surface of the substrate would occur but below that at which any appreciable amount of the metal halides formed would be removed therefrom by evaporation.
• this temperature usually lies within the range 400 to 700 0., preferably at about 600 C., though where aluminium is incorporated in substantial quantities in the initial coating, it may be necessary to employ temperatures as low as 180 C. to minimise the loss of aluminum halides from the furnace.
• the furnace is maintained at this temperature while the passage of non-oxidising gas is continued with the addition of a hydrogen halide.
• the amount of hydrogen halide in the gas stream and the rate of passing of the gas stream through the furnace are preferably adjusted so that there is little or no hydrogen halide in the furnace off-gases.
• the incorporation of hydrogen halide in the furnace atmosphere is continued until suflicient halogen has been fixed on the surface of the substrate to ensure that a satisfactory speed of chromising in the stage 3 of the process of the invention is achieved.
• the speed of chromising in stage 3 of the process in general increases with an increase in the amount of halogen fixed on the surface of the substrate, the use of two great an amount of halogen results in wastage of metals which would otherwise have formed part of the alloy coating since these metals are left as metal halides which are removed from the system at the end of the heating period.
• the amount of hydrogen halide incorporated into the chromium coating will therefore generally vary from about 30% to 60%, preferably 50 to 60%, of the theoretical amount of hydrogen halide required to react with all the chromium in the coating.
• stage 3 of the process the temperature of the furnace is raised to a temperature of at least 750 C. at which temperature chromising takes place and is held at this elevated temperature for as long a period as is required to alloy all the chromium in the initial coating with the ferrous metal and form an alloy coating of the desired thickness.
• a temperature of at least 750 C. at which temperature chromising takes place and is held at this elevated temperature for as long a period as is required to alloy all the chromium in the initial coating with the ferrous metal and form an alloy coating of the desired thickness.
• the thickness and composition of the alloy coating obtained from any given amount of chromium are controlled by the conditions, that is the time and temperature, under which chromising is achieved and the halogen content of the initial coating.
• the thickness of the alloy coating increases with an increase in these variables within the limits outlined herein.
• a corrosion resistant coating on mild steel having a thickness of 0.002 to 0.003 inch and containing approximately 30% chromium may be achieved by carrying out the chromising at 800 C. for a period of about 36 hours.
• coating containing 80 to 90% chromium and only 0.0005 inch thick may be obtained by carrying out the chromising at 750 C. f r approximately 36 hours.
• EXAMPLE 1 20 gauge 0.2% carbon steel sheet is degreased by immersion in an alkaline cleaner and is then treated anodically in 50% sulphuric acid for 30 seconds at a current density of 400 A./sq. ft. The sheet is transferred to 2. catalyzed chromic acid chromium plating bath and a current of 300 A./sq. ft. is passed until a layer of chromium 0.0003 in. thick has been deposited. The coated sheet is washed and dried and loaded into a suitable furnace. The furnace is purged with hydrogen and heated to 600 C. A quantity of chlorine equivalent to 60% of that theoretically required to react with the chromium in the coating is injected into the hydrogen purge, over a period of 6 hours.
• the flow of gases is then stopped and the furnace is heated to 800 C. and maintained at this temperature for 36 hours. After cooling, the sheet is removed from the furnace.
• the surfaces are silver-grey in colour and are resistant to corrosion by water, aqueous sodium chloride, aqueous nitric acid, etc., even after bending, etc.
• Removal of a portion of the coating by filing and treatment with 50% aqueous nitric acid to dissolve the steel core reveals a coating insoluble in nitric acid of 0.0009 in. thickness. Analysis of the coating after dissolution in hydrochloric acid shows an iron content of 71%.
• EXAMPLE 2 0.2% carbon steel sheet is pickled and degreased as in Example 1 and plated with chromium 0.0012 in. thick. The sheet is then heat treated as in Example 1 for 36 hours. A chromised coating 0.0021 in. thick, containing 52% Fe is obtained.
• EXAMPLE 3 20 gauge 0.2% carbon steel sheet is degreased and pickled as in Example 1 and plated with chromium 0.0004 in. thick. The sheet is then heat treated as in Example 1 except that the final stage of the heat treatment is one of 36 hours duration at 750 C. A chromised coating 0.0004 in. thick containing 12.7% iron is obtained.
• EXAMPLE 4 20 gauge 0.2% carbon steel sheet is degreased, pickled and plated with 0.0003 in. chromium. Portions of the sheet are chromised, using 33% and 47% respectively of the theoretical quantity of hydrogen chloride required to react with the chromium. The final stage of the heat treatment is one of 36 hours duration at 800 C. Coatings of thickness of approximately 0.0003 in. containing 3.3% and 10% iron respectively are obtained.
• EXAMPLE 5 20 gauge 0.2% carbon steel sheet is degreased electrolytically in an alkaline clearer and pickled in 10% v./v.
• chromium metal powder 200 BS mesh
• This is compacted onto the surface by passing the sheet between rolls.
• the strip is then chromised as in Example 1. On removing the sheet from the furnace it is found that a chromised coating 0.0034 in. thick containing 62% iron has been formed.
• EXAMPLE 6 gauge 0.2% carbon steel sheet is cleaned and pickled as in Example 5.
• a quantity of 200 mesh BS chromium powder is applied to the surface of the strip suflicient to give a coating after compacting by passing the strip between rolls of 0.00045 in. thickness.
• the steel sheet is then loaded into the furnace and after purging and heating to 600 C.
• a quantity of bromine equivalent to 80% of that required to react with all the chromium is injected with hydrogen into the furnace.
• the flow of gases is then stopped and heat treatment is carried out at 800 C. for 36 hours.
• a chromised coating 0.0008 in. thick containing 59% iron is obtained.
• EXAMPLE 7 0.8% carbon steel sheet is degreased and pickled as in Example 1 and plated with chromium to a thickness of 0.00015 in. The sheet is then chromised as in Example 1. A chromised coating 0.00043 in. thick containing 61.2% iron is formed.
• EXAMPLE 8 18/8 stainless steel sheet is degreased and pickled, plated with chromium to a thickness of 0.0003 in., and chromised as in Example 1. On removing the chromised stainless steel from the furnace it is found that enhanced resistance to high temperature oxidation has been obtained. On subjecting pieces of the original stainless steel, the chromium plated stainless steel and the chromised stain less steel to air at 800 'C. for 7 hours, the samples showed weight gains of 0.38, 0.04 and 0.00 g. per sq. ft. of surface respectively.
• a process for the chromising of ferrous metal substrates which comprises:
• a process according to claim 1 wherein the coated substrate is heated is contact with a stream of hydrogen or argon containing a hydrogen halide selected from the group consisting of hydrogen chloride and hydrogen bromide.
• a process according to claim 5 wherein the amount of hydrogen halide fed to the furnace is from 50% to 60% of the theoretical amount required to react with all the chromium in the coating on the substrate.
• a process for the chromising of ferrous metal substrates which comprises:
• a process for the chromising of ferrous metal substrates which comprises:
• a process for the chromising of ferrous metal substrates which comprises:
• a process according to claim 10 wherein the chromium powder applied to the substrate has a particle size of 75 microns or less and the coating is up to 0.003 inch thick.
• a process for the chromising of ferrous metal substrates which comprises:
• a porous adherent chromium-containing coating on the ferrous metal substrate by applying a powder selected from a group consisting of chromium and chromium alloys having a particle size of up to 75 microns and subsequently compacting this powder upon the substrate by means of a rolling technique, said coating being up to 0.003 inch thick;
• a process for the chromising of ferrous metal substrate and form an alloy coating of the desired strates which comprises: thickness.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Other Surface Treatments For Metallic Materials (AREA)
• Electroplating Methods And Accessories (AREA)
• Chemical Treatment Of Metals (AREA)

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Cited By (7)

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• 1965
  • 1965-07-01 GB GB27956/65A patent/GB1146594A/en not_active Expired
• 1966
  • 1966-06-23 DE DE19661521122 patent/DE1521122B1/de active Pending
  • 1966-06-29 BE BE683362D patent/BE683362A/xx unknown
  • 1966-06-30 FR FR67741A patent/FR1513264A/fr not_active Expired
  • 1966-07-01 AT AT629066A patent/AT263478B/de active
• 1969
  • 1969-09-02 US US856884A patent/US3589927A/en not_active Expired - Lifetime

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