US3152007A

US3152007A - Process for chromizing ferrous metal objects

Info

Publication number: US3152007A
Application number: US102652A
Authority: US
Inventors: Tom S Perrin; Douglas H Strong
Original assignee: Diamond Alkali Co
Current assignee: Diamond Shamrock Corp
Priority date: 1961-04-13
Filing date: 1961-04-13
Publication date: 1964-10-06
Anticipated expiration: 1981-10-06

Description

N m 0 R W T R S R Hm E P s A J L G M w I l I I I I I I I l I I I l l l l I l I I I l i I I O T D v M55376 558 $2135. 5 m d 532$ 923:6 It; $2.528 120.25%: mmwzamozz ESE T l1 \imwxmmozb W I m I I L1 a- 1 k MED. i 2954mm u n n mo z-5 Oct. 6, 1964 PROCESS FOR CHROMIZING FERROUS METAL OBJECTS ATTORNEY United States Patent 3,152,007 PROCESS FDR CEiOMiZlNG FERROU hdETAL OBJECTS Torn S. Perrin, 'iainesville, and Douglas H. Strong,

Willoughby, Ohio, assignors to Diamond Alkali Company, Cleveland, Ohio, a corporation of Delaware Filed Apr. 13, 1961, Ser. No. 102,652 7 Claims. (Ci. 117-107.2)

This invention relate to a process for protecting ferrous metal surfaces from corrosion and oxidation by forming a chromized surface thereon.

In the usual chormizing process, the object to be treated is brought into contact with vapors of a chromium chloride or with chromium formed by the reduction of these vapors, at a temperature of about 1500-1900 F. The chromium chloride vapors undergo an exchange reaction with the iron on the surface of the object, with the formation of metallic chromium and its diffusion into the surface. In addition, the elemental chromium formed by the reduction of the chromous chloride itself diffuses into the surface of the object being treated, the result being the formation of an alloy of high chromium content, resistant to corrosion and oxidation, on the surface of the object being treated.

Previously described chromizing procedures have generally fallen into one of two classes. The first of these employs elemental chromium which is converted to chroznous chloride by the action of hydrochloric acid or a carrier material such as ammonium chloride. This method suffers from the disadvantage that at the temperatures normally employed the vapor pressure of the reacting species, chromous chloride, is very low, about 0.0005 atmosphere. At this pressure, the formation of a chrornized layer of suitable thickness requires a long period of time.

In the second method, the chromium source is either chromous chloride or a mixture of chromium chlorides. At the reaction temperature the chromous chloride is vaporized and transmitted, either by a porous transmitting medium or by a gas, usually hydrogen, to the surface of the metal to be coated. The chromic chloride, if any, in the mixture is reduced by hydrogen to chromous chloride before vaporization and, as such, is transmitted to the metal surface. This method suffers from the same disadvantage as the previous one; namely, that the vapor pressure of chromous chloride at the temperatures employed for chromizing is quite low (0.0490 atmosphere at 1827 F.) and that the process is, therefore, very timeconsurning. Also, chromous chloride is a rather unstable compound which is difiicult and expensive to prepare and store.

It is the object of the present invention to provide an economical process for producing a chromized surface on a ferrous metal object. A further object of the present invention is to reduce the time now required for chromizing ferrous articles. These and other objects will become apparent to those skilled in the art upon reading the description of the process which follows.

In the drawing: The FIGURE is a diagrammatic illustration of an experimental apparatus used in this invention.

The method of this invention employs a chromic chloride, CrCl as a chromium source in the chromizing process. Chromic chloride has the advantage of being less expensive and easier to obtain and handle than chromous chloride and, in addition, has other advantages which are more fully described hereinbelow.

3,152,0h7 Patented Get. 6., 1964 "ice. A

The chemical reactions simultaneously involved when chromizing with the chromium chloride and hydrogen are as follows:

(1) CrCl +ll ?2CrCl 21-101 (2) CrCl +H Cr+2HCl (3) 2Fe(metal surface) +CICl e FeCr(surface case) +FeCl (4) FCIQ+HQZFB+ 21-101 (5) Fe (metal surface) Cr FeCr (surface case) A protective case formed on the metal surface is brought about when chromium difiuses into the metal surface as a result of Reaction 3 and when chromium is deposited on the metal surface as a result of Reaction 5. It has been estimated that about half of the case formed on the metal surface comes from Reaction 3 and half from Reaction 5. The thickness of the protective case formed by this process depends upon: (a) the prevailing temperature while the metal surface is exposed to the chromium chloride and hydrogen; (b) the vapor pressure of the chromium chloride; (0) the ratio of H and HCl during chromizing; (d) the time of exposure of the metal surface at chromizing conditions.

The temperature is important because it determines the vapor pressure or" the chromous chloride and also the rate of didusion of chromium into the metal surface. Normally, temperatures of 13002000 F. are used.

The vapor pressure of chromous chloride is determined by the mode of conducting the chromizing process. The vapor pressure of chromic chloride is much higher than that of chromous chloride as shown hereinbelow; therefore, if hydrogen is not mixed with CrCl until it is vaporized, the CrCl vapors can be reduced to give CrCl at the same vapor pressure. This is the process which has been used in the present invention.

CrOI atm. GrCl atm,

The amount of hydrogen used, or the hydrogen flow, dete mines the HChl-l ratio, and this ratio determines the extent to which the equilibrium Reactions 1-4 take place. At the temperatures used in the chromizing process, Reaction 1 goes essentially to completion. The HCl thus formed suppresses Reactions 2 and 4, and, therefore indirectly suppresses Reaction 3 by decreasing the rate of removal of FeCl from the system by Reaction 4. A balance must, therefore, be maintained between a rapid hydrogen flow, which would sweep the HCl out of the system and promote Reactions 2 and 4 but would also sweep away appreciable quantities of CrCl vapor, and a slow flow, which does not sweep out the CrCl but also leaves HCl in the system and retards Reactions 2 and 4. it has been found that a replacement rate per hour of about 0.5-1.0 mole of hydrogen per mole of chromic chloride used is optimum.

The experimental apparatus used in this invention and illustrated diagrammatically in the drawing consists of a furnace through which runs a horizontal tube with an inlet for gas at one end and an outlet at the other end. The furnace should be suitable for heating the chromizing chamber to a maximum temperature of about 2000" F. In the approximate center of the tube, in the heating zone, is placed a vessel containing chromic chloride, fitted with a perforated cover allowing for diffusion of the chromic chloride vapors but excluding most of the hydrogen to prevent reduction of the solid CrCl Samples to be chromized are suspended above this vessel. It is also desirable to provide a means for thorough mixing of the hydrogen and chromic chloride vapors.

The system is swept with an inert gas, usually dry nitrogen, helium or argon, while the temperature is raised to about BUT-1550 1 preferably 1400-l500 F. At this temperature, the atmosphere is changed to dry hydrogen, adding about 0.51.0 mole of hydrogen per mole of CrCl as quickly as possible and thereafter replacing the atmosphere with hydrogen at a rate of approximately 0.5-1.0 mole of hydrogen per mole of CrCl per hour. The system is maintained at chromizing temperature,

usually approximately 15002000 F., for 1-2 hours for about a 2 mil coating. The system is then cooled as quickly as possible to about 1000 F. while continuing the hydrogen flow. At this temperature the hydrogen is replaced by the inert gas and the temperature is cooled to about 200 F.

The following example is given in order that those skilled in the art may more completely understand the invention and the means for carrying the same into effect.

Seventy-five grams of chromic chloride is placed in a vessel with a perforated cover and placed in the heating zone below the samples to be chromized. In an argon atmosphere, the vessel is heated to 1500 F. The atmosphere is converted to hydrogen at this point. The

rate of addition or" hydrogen is controlled by igniting the exhaust gases and adjusting the flow so that the flame is approximately %-inch long. (Alternatively, a fiowmeter may be used to measure the hydrogen flow.) Rapid hydrogen chloride evolution begins with the introduction of hydrogen. perature reaches 1800 F. It is held at approximately 1860 F. for 2 hours. The hydrogen flow is increased slightly and the vessel is cooled to 950 F. over a minute period. At this temperature the evolution of hydrogen stops and the atmosphere is reconverted to argon. After cooling to 200 F., the samples are removed. After sectioning, polishing and etching with 2:1 nitric acid solution, the samples are shown to have a chromized coating varying in thickness from 0.0020.003 inch. This coating does not react with boiling 20% ntric acid.

It is to be understood that, although the invention has been described with specific reference to particular em bodiments thereof, it is not to be so limited, since changes and alterations therein may be made which are within- After 2 hours and 40 minutes, the fem the full intended scope of this invention as defined by the appended claims.

What is claimed is:

l. A process for chromizing ferrous metal objects comprising the steps of heating the object to be chromized and chromic chloride in an inert atmosphere to a temperature such that the vapor pressure of chromic chloride is at least about 0.015 atmosphere, said chromic chloride being contained in a vessel with a perforated cover; passing hydrogen through this system to effect reduction of a vaporized chromic chloride to form gaseous chromous chloride and chromium at approximately the same vapor pressure; and contacting the surface of the object to be chromized with said gaseous chromous chloride and chromium.

2. A process for chromizing ferrous metal objects comprising the steps of heating the object to be chromized and chromic chloride to a temperature of about 1300- 1550 F. in an inert atmosphere, said chromic chloride being contained in a vessel with a perforated cover; passing hydrogen through the system at a temperature of about 15002000 F. to effect reduction of the vaporized chromic chloride to form gaseous chromous chloride and chromium at approximately the vapor pressure of the chromic chloride; and contacting the surface of the object to be chromized with said gaseous chromous chloride and chromium.

3. A process for chromizing ferrous metal objects comprising the steps of heating the object to be chromized and chromic chloride to about 1400-1500 F. in an inert atmosphere, said chromic chloride being contained in a vessel with a perforated cover; passing hydrogen through the system at a temperature of about 1800 1900" F. to effect reduction of the vaporized chromic chloride to form gaseous chromous chloride and chromium at approximately the vapor pressure of the chromic chloride; and contacting the surface of the object to be chromized with said gaseous chromous chloride and chromium.

4. The process of claim 3 wherein the inert atmosphere is nitrogen.

5. The process of claim 3 wherein the inert atmosphere is argon.

. 6. The process of claim 3 wherein the inert atmosphere is helium.

7. The process of claim 3 wherein the hydrogen flow rate approximately 0.5 mole of hydrogen per mole of CrCl per hour.

References Qited in the file of this patent Powell: Vapor Plating (1955), John Wiley (N.Y.), pp. 47 and 48 relied on.

' Insausti: Chromizing of Steel, translated from Instituto del Hierro y del Acero, vol. 9, March 1956, No. 44, pp. 250257 (p. 5 of translation relied on).

Claims

1. A PROCESS FOR CHROMIZING FERROUS METAL OBJECTS COMPRISING THE STEPS OF HEATING THE OBJECT TO BE CHROMIZED AND CHROMIC CHLORIDE IN AN INERT ATMOSPHERE TO A TEMPERATURE SUCH THAT THE VAPOR PRESSURE OF CHROMIC CHLORIDE IS AT LEAST ABOUT 0.015 ATMOSPHERE, SAID CHROMIC CHLORIDE BEING CONTAINED IN A VESSEL WITH A PERFORATED COVER; PASSING HYDROGEN THROUGH THIS SYSTEM TO EFFECT REDUCTION OF A VAPORIZED CHROMIC CHLORIDE TO FORM GASEOUS CHROMOUS CHLORIDE AND CHROMIUM AT APPROXIMATELY THE SAME VAPOR PRESSURE; AND CONTACTING THE SURFACE OF THE OBJECT TO BE CHROMIZED WITH SAID GASEOUS CHROMOUS CHLORIDE AND CHROMIUM.