Chromizing of Iron
Difuzijsko kromanje železa
M. Jenko, A. Kveder, Inštitut za kovinske materiale in tehnologije, Lepi pot 11, 61001 Ljubljana S. Spruk, L. Koller, IEVT, Teslova 30, 61111 Ljubljana
In the paper, the theoretical aspects of CVD processes of iron chromizing and the comparison vvith the PVD process, developed by the authors for professional electronic industry are presented.
V sestavku so podani teoretični vidiki CVD postopkov difuzijskega kromanja železa in primerjava s PVD postopkom, ki so ga ovtorji razvili za potrebe profesionalne elektronike.
1	Introduction
Corrosion is one of the most frecjuent and the most unde-sired processes on the surface of metals and alloys. Since corrosion is a surface reaction, ali types of protective coat-ing must be involved to change the behaviour of metallic component in the surface composition. This change can be achieved by addition of a different material or in the form of outer skin, vvhich provides a barrier betvveen the body and the surrounding corrosive medium. The form of coating is the most common; it includes paints, plastics, metals deposited by electroplating etc. It is also possible to modify the chemical composition of the surface to be pro-tected, by diffusion of a suitable metal or an element into it vvhich vvill combine vvith the parent metal or alloy and pro-vide the required resistance to the corrosive medium. Such formed surface alloys are called diffusion coatings. The di-mensional change of the protected specimen is smaller than the thickness of the effective surface alloy and it may be neglected.
Chromium diffusion—chromizing is probably one of the most versatile types of diffusion coatings and it is applied to achieve resistance to corrosion, thermal oxidation and abrasion for iron, steel, stainless steel, nickel and its alloys, molybdenum, tungsten and its alloy, etc.
2	Technological development of chromizing
The first attempts to achieve a chromium rich surface on iron by the diffusion process vvere made by Kelly in 1923". Iron specimens vvere buried into chromium povv-der and treated in reducing atmosphere. A chromium rich layer, about 125 /tm thick, formed after 4 hours heating at 1300°C, vvas a protecting layer vvith good adherence to the underlying metal, resistant to corrosion, as vvell as to thermal oxidation and, therefore, very interesting for vvide commercial use.
Similar investigation vvas made by H.S. Cooper in 19382. The process of chromizing vvas applied in a re-ducting atmosphere at the temperature of 1300°C, lasting 24 hours; the thickness of chromized layer vvas 250 /im.
A high processing temperature vvas disadvantage of both procedures.
The chromizing process has undergone considerable development changes over the years and it has been the subject of careful and detailed studies. A major achivement vvas
the introduction of volatile halides. L.H. Marshall developed the first CVD (chemical vapour deposition) procedure of chromizing, using the volatile halides at the processing temperature of 1050° C.
Modern chromizing processes like DAL, BDS, etc. are based on the above mentioned principle3,4.
Simultaneously, the first experiments of vacuum diffusion chromizing vvere performed by Hicks as early as 1932. Particles of pure iron vvere buried in a chromium povv-der and heated for 96 hours at 1200° C in a vacuum of 4 • 10"2 mbar. Eight years later, Cornelius and Bollenrath obtained similar results in their experiments; chromium con-centration profiles vvere determined by the X-ray analy-sis. Further, this process vvas described by Gorbunov and Dubinin12.
In Slovenia the vacuum chromizing process (PVD— physical vapour deposition) has been developed at the Institute for Electronics and Vacuum techniques together vvith the Institute of Metals and Technologies4, 17 and has been used for diffusion chromium coating of iron parls of mag-netic circuit for miniature relays.
3 Fe-Cr constitution diagram
The iron-chromium constitution diagram is shovvn in Figure 1. At approximatelly 1000°C, it can be seen that the austenite microstructure of the iron remains unchanged until a con-centration of approximately 12% chromium is reached vvhen chromium is deposited and it diffuses invvards. At higher chromium concentrations, the microstructure becomes ferritic; continuation of chromizing causes moving of the al-pha/gamma boundary into interior. During cooling the ferrite surface layer remains unchanged, vvhile inner austenite is transformed into ferrite. This recrystallization of inner region vvith less than 12% Cr causes that boundary vvith 12% Cr is good visible, Figure 2. The depth to vvhich extends the 12%- Cr boundary is taken as the thickness of chromized layer, Figure 3.
Since the rate of diffusion of chromium is greater in ferrite than in austenite, there is a rapid rise in the chromium concentration of the coating tovvards the surface, and be-yond the 12% Cr boundary there is a sharp concentration drop at the ferrite/austenite boundary. Grain boundary diffusion occurs too, but it has a little effect on the coat thickness, Figure 4.
1900 1800 1700 1600
Figure 3. Microsections of vacuum chromized iron samples at: 1050°C (a) 3 hours, (b) 8 hours (c) 12 hours, 1100°C (d) 3 hours, (e) 8 hours, (f) 12 hours, 1150°C (g) 3 hours. (h) 8 hours. Nital etched.
Slika 3. Metalografski posnetki vakuumsko kromanih vzorcev želeZa pri: 1050°C (a) 3 ure, (b) 8 ur (c) 12 ur, 1100°C (d) 3 ure. (e) 8 ur, (0 12 ur, 1150°C (g) 3 ure, (b) 8 ur. Jedkano z nitalom.
Effective coating
o o
1500 1400 1300 1200 1100 1000 900 800 700 600
3NS:
__i* i_i_* i——-—
0 10 20 30 40 50 60 70 80 90 100
% Cr
Figure 1. Iron—Chromium diagram5. Slika 1. Fazni diagram Fe—C5.
1 MRG: 750 IBIDIFF CR 1B,
Figure 2. a) Micro-section of vacuum chromized sample. A sharp o/phase boundary is visible (nital etched). b) Cr concentration profile of the same sample. Slika 2. a) Metalografski posnetek vakuumsko kromanega vzorca železa; vidna je ostra fazna meja alfa/gama (jedkano z nitalom). b) Koncentracijski profil kroma posnet z elektronskim mikroanalizatorjem na istem vzorcu.
The alloyed layer is generally called a coating, but it must be clearly distinguished from the eoatings produced by electroplating and spraying processes, since there is no diffusion. The chromized coating represents an inseparable part of the treated specimen, the composition is changing from the surface to the core.
4 Mechanism and kinetics of cromizing the iron
4.1 Chromizing technic/ue with volatilc halidcs
In these processes chromium is brought to the surface of the iron heated to 900-1150° C as a gaseous compound, e.g. chromium chloride, where it is deposited in atomic form by a chemical reaction.
O
12%
a
y
Distance from surface
Figure 4. Chromium concentration profile in o-FeCr layer.
Slika 4. Gradient kroma v kromani plasti.
In many chromizing techniques, chromium chloride is applied and an atmosphere containing hydrogen is main-tained in the reaction chamber.
The deposition of chromium on iron is described vvith the follovving equations:
Interchange
Reduction
Fe + CrCb = FeCl: + Cr
CrClj + H2 = 2HC1 + Cr
• Dissociation
CrCl2 = Cl2 + Cr
(D
(2)
(•'I)
In the reaction (1) an atom of iron is removed from the surface for each deposited chromium atom. Since iron and chromium atoms are similar in vveight and size, there occur
only slight mass and dimensions changes of iron specimens after the treatment. The reaction is reversihle and the equi-librium chromium concentration at the surface depends on the relative vapour pressures of iron and chromium chlo-rides in gaseous phase.
Reactions (2) and (3) are catalysed by the iron surface. Theoretically the surface chromium concentration may ap-proach 100 per cent, but since it is assumed that the cat-alytic activity of the iron surface drops with the increasing chromium content, the concentration of chromium is limited. The mass and dimension change are equivalent to the amount of deposited chromium.
Generally, the volatile halides are used for transport of chromium atoms to the surface of iron, where they are ad-sorbed and diffuse imvards.
In Figure 5 the layout of BDS (Becker, Daeves, Stein-berg) proeess, a typical CVD proeess, is shovvn.
2. Chromium migration from the surface inwards into the specimen expressed by the interdiffusion coefficient D:
D = 2.08exp(-243000/RT)
(7)
Termoregulation
Oegassing
Sample
Figuro S. The layout of BDS (Becker. Daeves, Steinberg) proeess, typieal CVD proeess, is shovvn. Slika 5. Shematičen prikaz CVD-BDS postopka.
(b)
Vacuum chromizing
There are tvvo possible processes of supplying an iron surface vvith the chromium atoms:
•	transfer due to the close contact of iron surface and chromium granulate enabling the surface diffusion of Cr
•	absorption of Cr vapour through the formed gaseous phase
In vacuum chromizing the grovvth of a — FeCr layer is controlled by tvvo processes:
1. The arrivai and condensation of C r atoms on the surface of the specimen given by the condensation rate w:
(4)
/g cm-2s"7(5)
vvhere ak is the condensation coefficient; pCr (mbar) is vapour pressure; M is Cr molecular mass and T is absolute temperature (K). The decisive quantity is pcr, and its temperature dependence is being described by
p = 11.743exp(-394000//žr) /mbar/ (6)
(R is the gas constant in JK-1mol-1)
0
2
10
12
14 t/h
Figure 6. Thickness of the chromized layer, d, and the mass inerease, W, as a funetion of chromizing time t. a) experimental results b) calculated values. Slika 6. Debelina vakuumsko kromane plasti d in narastek teže W v odvisnosti od časa t a) eksperimentalni rezultati b) izračunane vrednosti.
By increasing the temperature 7', p inereases more rapidly than D as the evaporation enthalpy of chromium AHevap = 394 kJmor1 is higher than
the aetivation
energy for the diffusion Edij = 243 kJmol-1. This circumstance leads to three dif-ferent a-FeCr layer grovvth rates.
(a)	At lovv temperatures 950 < 6 < 1050°C the slovvest proeess is the Cr condensation. Ali con-densed Cr atoms are transported immediately by diffusion from the surface invvards. Therefore the layer grovvth rate is linearly proportional to the condensation rate w:
w = Dt or d = Vwt
(b)	At high temperatures 0 > 1150°C, vvhen p is high enough, the slovvest proeess is the diffusion, leading to the parabolic law
w = Dt
d = vVTDt
w
(Trn
0 2	8 10 12 K t/h
Table 1. Values applied in the evaluation of the thickness. d, and ueight inerease, W, of u-FeCr layers given in Figure 6b. D-interdiffusion eoeffieient, p-equilibrium Cr vapour pressure, u>-condensation rate of chromium, ta, da—critical time eoresponding o-FeCr layer thickness
when the linear growth rate changes into the parabolic one.
°C	900	950	1000	1050	1100	1150	1200	1250
D (cm1,-1)	1.52 x 10-11	4.20 X 10~u	1.07 x 10"10	2.56 x 10~10	5.72 X 10-10	1.21 x 10"9	2.43 x 10~9	4.65 x 10-9
p (mbar)	1.0 X 10~7	8.0 X 10"7	3.8 x 10-6	1.5 x 10-5	5.7 X 10"5	1.9 X 10"4	5.9 x 10"4	1.7 x 10-3
W (gcm-2s-')	9.23 X tO"10	7.24 X tO"9	3.36 X 10"8	1.3.5 x 10"7	4.88 X 10-7	1.62 X 10"6	4.8S X 10"6	1.36 x 10-3
ta (S)	4.32 x 107	1.94 X tO6	2.3 x 105	3.4 x 104	5829	1150	247	59.31
(h)	1200	538.8	63.9	9.52	1.6	0.319	0.069	0.0165
da (^m2)	362.4	127.60	70	41.9	25.8	16.6	10.9	7.3
At these values of T the linear rate appearing in the early stages of growth cannot, be detected.
(c) In the intermediate region 1050 < 0 < 1150°C the thickness of o-FeCr layer begins to inerease linearly with time. The grovvth rate changes to a parabolic low at the critical time t, which corre-sponds to the critical thickness d.
The calculated grovvth rates of a-FeCr layers are shovvn in Figure 6b. Table 1 contains ali necessary data; p is obtained from the equation (6), w from the equation (5) assuming a — 1, D from the equation (7), and da from the relationships given in Figure 6a.
! GORBUNOV ? KUBASCHEVSKI 3 BRf WER PAZUHIN
5	KOVALENKO
6	THIS W0RK
7	H 0 NIG
Preal = CiC2C3p
(8)
vvhere: c i
co	takes in account the residual atmosphere;
c3	takes in account the surface/granulate ratio.
5 Conclusion
The results of this investigation shovv that none of CVD processes is suitable for the proteetion of iron parts for mag-netic circuit in miniature hermetic relays. For this purpose PVD process of vacuum chromizing vvas developed. With this procedure the maximal chromium eontent of 15% Cr at the surface vvas obtained, enough for corrosion proteetion in corrosive media vvhich are demanded by MIL-R-39016 and MIL-R-5757. PVD process assures the optimal magnetic properties, a very lovv coercivity and a good weldability and additionally, it is an environment friendly process.
1000 1/00 '100 v. .10 1800
Figure 7. Temperature dependence of equilibrium Cr vapour pressure p according to various references"-16.
Slika 7. Pami tlak kroma v odvisnosti od temperature po podatkih različnih avtorjev11-16.
The real Cr vapour pressure preai is equal to the equi-librium pressure p only if the experimental conditions are carefully chosen: pra < 10"4 mbar, vvhile the surface of the chromium granulate has to be as clean as possible and the ratio of specimens surface and the granulate amount must be adequate. If these conditions are not correctly chosen then prcai can be expressed by p multiplied by three correction coefficients c i, C3 < 1:
A —tXI
H		v-
		
1		|i____|
		
-1		
1	dvostopenjska rotacijska črpalka
2	rr.enlnik srednjega vakuuma
3	turbomolekularna črpalka
4	■ ventil
5	- merilnik visokega vakuuma b - izolacija
7	grelec
8	škatla iz nerjavnega jekla
9	kremenčeva cev
takes in account the portion of oxidized surface of granulate;
Figure 8. A schematic diagram of PVD—vacuum chromizing procedure7.
Slika 8. Shematičen prikaz PVD—vakuumskega difuzijskega postopka kromanja.
6 References
1	N.A. Lockington, in Corrosion, Vol. 2, Principles of Applying Coatings by Diffusion, Buttervvorths, London 1978.
2	R.L. Samuel, N.A. Lockington, Met. Treat. 18. 354. 407, 440 (1951).
3	G. Becker, K. Daeves, F. Steinberg, Stahl und Eisen 61. 289 (1941).
4	M. Jenko, A. Kveder, R. Tavzes, E. Kansky, J. Vac. Sci. Technol. A3. 6. 2657(1985)

E. Kansky, M. Jenko. Vaeuum 37, 1/2, 81 (1987).
M. Jenko. R. Tavzes. E. Kansky. J. Vac. Sci. Technol A5/IV 2685 (1987).
A.H. Sully. E.A. Brandes, Chromium, Chpt. 7, 258, But-terworth. London (1967).
H. Cornelius, s. R Bollenrath, Arch. Eisenhuttenwesen, 15. 145 (1941).
L.C. Hicks, Trans. AIME. 113. 13, 163 (1934). T. P. Ho ar. EA. Croom. J. Iron Steel Inst. 169, 101 (1951). R.E. Honig, D.A. Kramer. RCA Rev. 30. 285 (1969). N.S. Gorbunov. Diffusion coatings on iron and steel. Is-rael program for Sci. translations. Jerusalem 1962.
13	V.A. Pazuhin, A.J. Fišer, Vakuum v metallurgii, Metal-lurgizdat, Moskva (1956).
14	V.E Kovalenko, Teplofizičeskie Procesy i Elektrovaku-umnye Pribory, Sovetskoe radio, Moskva (1957).
15	J.L. Margrawe, The Characterization of High Temperature Vapours, John Wiley and Sons, New York (1967).
16	O. Kubaschewski, E.L. Evans, C.B. Alcock, Metallurgi-cal Thermochemistry, Pergamon, Oxford (1967)
1' M. Jenko, A. Kveder, R. Tavzes, Postopek za protikorozi-jsko ščitenje majhnih kosov iz čistega železa ob hkratnem doseganju najboljših mehkomagnetnih lastnosti. Številka patenta: P-19470.