Kinetics of aluminum chromating

by M. Hassib Abbas, National Research Center, Dokki, Giza, Egypt

ne of the more practical and important factors affecting and w controlling the kinetics of the chromating process is the temperature. Many studies1’2 have been undertaken to test this factor. Chromating temperatures vary from ambient to boiling, depending on the particular solution and metal being processed. The formation of a gelatinous deposit at room temperature proceeds rather slowiy and the coating is relatively thick. For a given system, an increase in the solution temperature will accelerate the film-forming rate to yield a thin coating, as well as the rate of attack on the metal surface. This can result in a chromate film that is hard and possesses lower adhesion to the substrate. Thus, temperatures should be adequately maintained to ensure consistent results.3 In spite of many studies of temperature effect on the chromating rate, little attention has been paid to investigating the kinetics of the chromating process.

To study the influence of bath variables on process kinetics, certain conditions were varied while others were held constant. The variables tested included bath temperature, sodium dichromate concentration, potassium fluoride concentration, chromating time, bath conductivity, specimen surface area to solution volume ratio, and agitation speed. Table I gives the process parameters . .* ” _. ana tne range or me c’hromating conditions. The chromating rate was taken as the coating weight per unit area of the specimen. Determination of the coating weight can be accomplished as follows:4

EXPERIMENTAL

The loss in weight (Am) was found from the equation:

The chromating experiments were rar&rl vu. nllt with ?il .,L.LIA”.. ,,a... I”

ml ..-

nf V. the Y.” bath “I...

contained in a beaker. The beaker was placed in a water bath. The method of heating was based on a hot plate, provided with a variable-speed magnetic stirrer to attain solution agitation. To adjust the temperature, a contact thermometer was used with temperature control within 2 1°C. All test runs were conducted on aluminum test specimens (20 cm2) of 94.66% aluminum and 5.17% silicon. The test specimen preparation was as follows: 1. 2. 3. 4. 5. 6. 7. 8. 8

Polish. Scrub with calcium carbonate. Rinse. Degrease with acetone. Rinse. Chromate process. Rinse. Air dry.

Weigh the chromated aluminum specim_en fm.1 nf knnwfi sgrfzce area (A). “--” -_ Dissolve in aqueous solution containing 7% (Wt/Vol) sodium hydroxide until the coating is removed. After treatment, rinse, dry, and weigh (m,).

Am=m,

-m,

(1)

The weight of the coating per unit area (g/m2) was then calculated from the equation: mA = l,OOOm/A

RESULTS AND DISCUSSION I ne relationsnip between temperature and other parameters affecting aluminum chromating kinetics can be summarized as follows. The effect of sodium dichromate concentrations on the coating weight at different temperatures is shown in Figure 1. At a temperature interval between 30 and 4OC, the conversion was mnid a-I._,

fnrmino A_ . . . ..I. a .Q thick u......

rnrrtino _‘..““~

nt .s. lnw .V I.

sodium dichromate concentration. Increasing bath temperature and sodium dichromate concentration above 4OC and 3 g/L, respectively, caused a decrease in the coating weight, which may be due to a decrease in the pH of the solution as a result of increasing sodium dichromate concentration, as well as to the dehydration of the chromating layer. Figure 2 shows a linear relationship between the coating weight and potassium fluoride concentration at various temperatures. This can be attributed to the activation properties of the potassium fluoride. It is worth noting that the increase of coating weight at temperatures between 30 and 40°C was relatively higher than at the temperature interval 40-XX at all concentrations; this was explained by higher depletion of potassium fluoride at elevated temperatures. The dependence of coating weight on both time and temperature is de

(2)

I ’ ’ ’ ’ ’ ’ ’ I

where m is given in mg and A in mm*. To improve accuracy, each experiment was repeated three times and the weight average was evaluated. Xttc acid was used to adjust the bath conductivity. All chemicals used were of reagent grade. Table I. ChromatingConditions Bath temperature, “C Sodium dichromate concentration, g/L Potassium fluoride concentration, g/L Time, minutes Bath conductivity, pmohskm x lo-* Area/volume ratio Agitation speed, rpm

0 Copyright

Elsevier Science

30-50 l-7 0.26-l .o 5-20 1B5-6.3 1:1.5-1:6 O-750

Inc.

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1

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Sodium dichromate dihydrate, g/L

Figure 1. Effect of sodium dichromate concentration and temperature on coating weight. (KF = 1 g/L, 10 min, 3.5 pmohslcm x lo-*, area/volume ratio. = 1:1.5, 0 rpm.) METAL FINISHING

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1995

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2-

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‘g&

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0 b

Potassium Fluoride, g/L

Bath Conductivity micromohkm x I 0-2

Figure 2. Effect of potassium fluoride concentration and temperature on coating weight. (Na,Cr,O,*2H,O ?? 3 g/L, 10 min, 3.5 p mohskm x 10T2, area/volume ratio ? ?lA.5,0 rpm.)

Figure 4. Effect of bath conductivity and temperature on coating weight (10 min, other parameters as in Fig. 3.) Figure 6. Effect of agitation speed and ----u*..u __ ---.I-...-L-L. I~---... rn~~)ararura VII waw~y waiyn~ \rarameters as in Fig. 4.) ??

pitted in Figure 3. At all temperatures under investigation, and between 5 and 10 minutes, the graph showed an initial rapid increase in the coating weight, followed by a decreasing rate until it reached the lowest value at 20 minutes. The decrease of the coating weight with increasing time (after 10 minutes) can be explained by consumption of bath components and increase in the pH of the solution. It is essential to emphasize the importance of bath conductivity as a factor affecting the chromating process (Fig. 4). At all tested temperatures, there was an increase in the coating weight at values of conductivity between 1.85 and 3.5 pmohs/cm X 10e2. Thereafter, a considerable decrease in coating weight was attained till it bePl?nP ‘7CVA at u9 WalllP nf ““aaY..“CI. mnrl11rtivitvS&J“I nf .,_a11 __I” UC . UlUY “I 6.3 pmohs/cm X 10V2 due to a decrease of the pH with the increase of conductivity. At 5O”C,the drop in coating weight was more than at 30 and 40°C because the corrosion rate was faster than the rate of deposition and,

I

Figure 5 indicates the relationship between the coating weight and both area/volume ratio and temperature. The decrease in the coating weight at different area/volume ratios and temperatures, as well, was a result of hydrogen ion consumption in the solution due to the higher reactivity of aluminum. This phenomenon was observed more at 50 than at 30 and 40°C. When dealing with the effect of agitation, a slight increase in coating weight with speed of agitation at temperatures under investigation is reported (Fig. 6). A possible explanation for the negligible effect of agitation at tested temperatures, which is a diffusion-controlled process (as will be seen later), is that the velocities employed tin ‘Andin 111tha U&Unorcitro+d pac.uI.ucIu

/--“: 2-Q I

1

5

10

15

I 20

1

Time, min.

Figure 3. Effect of time and temperature on coating weight. (Na2Cr20,-PH,O ? ?3 g/L, KF = 1 g/L, other parameters as in Fig. 2.) 10

0, 0

chromating kinetics, it can be said that the decrease in the coating weight above 40°C is explained as follows: (1) dehydration of the chromated layer; (2) increasing the temperature is accompanied by either an increase or decrease in the pH of the bath according to the treated parameter; or (3) depletion of the bath components is earlier at temperatures higher than 40°C.

maninn* hnmrntrar it lrg’““, A‘“I.1.II) 1c

is known that in the case of diffusioncontrolled processes, by increasing agitation gradually the metal substrate undergoes an active to passive transition.5 On the other hand, the slight im-

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provement in coating weight can be due to the solution replenishment and the elimination of hydrogen bubbles at the specimen interface. Based on the results obtained as to the influence of temperature upon the parclmerers _r_C___L__~affecting the aluminum

PROCESS KINETICS This section deals with developing empirical rate correlations, as well as with the kinetics of the chromating process. The aforementioned data were used for each parm,eter at different temperature intervals, i.e., 30-40 and 40-50°C. Empirical

Rate

Correlation

Empirical relationships to correlate the coating weight of the chromating layer (reaction rate) with the operating parameters can be described by a general equation of the form: I 1 : 1.5

I 1:3

I 1: 4.5

I 116

SurtaceNolume Ratio

Figure 5. Effect of surface area/volume ratio and temperature on coating weight. (Parame ters as in Fig. 4.)

W = K (X,)” QQb (X3)’ (X.Jd (X,)” (Uf

(1)

where W = coating weight (g/m’) and K = reaction constant. METAL FINISHING

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1995

Table II. Kinetic Parameters and Reaction Orders of the Chromating Process

1 g/L r X2 = 0.25 g/L 20 min 2 X3 1 10min

Temperaturelnterva/(IT) ReactionOrder

Range of Operating Parameters 1-34/L 3-7gll nnc 4 -,I “La-l

yfl.

5-10 min 10-20 min 1.85-3.5 lmohskm x lo-* 3.5-4.9 pmohekmx lo-* 25-750 1:1.5-1.6 rpm 250-750 rpm

Resides __L____ the ___ temnerature. _____r-------7

&_ere are

six variables affecting the coating weight, which are: X,-Sodium dichromate concentration, g/L X,-Potassium fluoride concentration, g/LChallenge Inc X,-Time, minutes X,-Bath conductivity, p mohskm X lo-’ X,-Area/volume ratio X,-Agitation speed, rpm To obtain the reaction orders a, b, c, d, e, and f, the coating weight was plotted against the aforementioned variables on a log-log scale and their values were computed from the slope ,.F G4b‘ ,.,,.I. lLUI _.._,a ,&.,:,*,~ :.. TT “I “6 _n (w UGrJlGLGU 111Tnl.la la”,= II. Table II indicates that the reaction orders are divided into two categories: Positive Reaction Order This shows an increase in the rate of reaction at a definite range of the operating parameters. The developed empirical correlations were obtained by substitution of these reaction orders into equation (1) and can be written for the operated temperature intervals as follows: 3&-4o”C W = K(X,)“.14(Xz)o.32(X3)0.6

30-40

40-50

0.140 -0.220 03220 0:SOO -0.320 0.290 -0.740 -0.175 0.031 0.075

0.140 -0.220 0.3% -Kl 0:250 -0.600 -0.135 0.027 0.064

Based on these empirical relationships, some conclusions may be summarized: 1. The utmost factor affecting coating weight of the chromating layer is the time. 3I. I-% II&” pnatino ‘“UC”‘6 w&oht . ..#“6aw ;r n.3mnrn ‘Ia”IY rlcanan_ uvpardent upon potassium fluoride concentration, bath conductivity, and sodium dichromate concentration, respectively; less dependent upon agitation speed; and independent of the area/volume ratio. 3. The negative values of reaction orders show unfavorable effects on the rate of reaction; therefore, the orders in this case can be neglected and their empirical correlations are of no value.

35 umohskm ~.-------.____X 10V2 2 X4 Z 1,85 pmohskm X 10m2 1:6 area/volume ratio 2 X5 2 1:1.5 area/volume ratio 750 rpm 2 X, B 250 rpm Equation (2) represents the empirical correlation for the optimum operating conditions. Negative Reaction Order This means a decrease in the reaction rate. The developed empirical correlations are explained in the next equations for the following temperature ranges:

Activation-Energy and FreeEnergy Computations The temperature dependency of reaction was determined by the activation energy from Arrhenius’s equation:

30-4OYJ W w K(X~)-“~22(X*)o~32(X3)~o~32 (x4)o.74(x5)-o.175(x6)o.o75 (4) *_ -,.a*50-C W p K(Xi)-“‘22(X*)0’30(X3)-0.27 (x4)~~~(x5)--o~~~~(x6)~~~~ (5)

K = AeemT log

7 g/L 2 x, 2 3 g/L

12

(7)

R = Universal gas constant, 1.99

Table Antivatinn.Fnernu .“I.” III ...._“...“““.. _..v. aJ IFI \-, anrl I.._ FmaFnarnv . .- “‘W.~, IAW \-.., Valuar 1...___IKcalln \..__YJ mnl\ ..._.I at . . Different _..._._...

Parameter Ranges Temperature Interval (“Cl 30-40 40-50

Sodium dichromate, g/L

Parameter Range 2

E

E

AH

AH

1.60 1.83

0.70 0.70

-5.27 -5.50

-5.72 -5.26

0.25-l

1.83

0.70

-4.61

-5.26

5-10 lo-20

1.60 2.50

0.70 1.37

-5.27 -5.50

-5.95 -5.95

Bathconductivity, p mohskm x10-*

1x-3.50 3.5w.9

1.63 2.76

0.70 1.83

-5.05 -5.00

-5.26 -5.72

Area/volume ratio

1:1.5-1:6

2.00

1.14

-4.60

-5.03

Agitation speed, rpm

25750 250-750

1.50 1.60

0.916 1.14

-5.53 -5.50

-6.18 -6.18

4%50°C Potassium fluoride, g/L W = K(Xi)“~14(X2)o~30(X3)0~60(X4)o~25 (x5)-o.135(x6)o.O64 Time, min. (3)

3g/Lrx,r1g/L 1 g/L 2 x, 2 0.25 g/L 10 min 2 X3 2 5 min

K = log A - E/2.303X l/T

E = Activation energy, caVg mol

(2)

These equations are valid within the following range of the operating conditions:

(6)

where K = Chromating rate, g/m*

and are valid within the following ranges:

Parameter

(x4>O_29(,5)-0.175(,6)0.075

4.9 pmohskm X 10e2 2 X4 2 3.5 pmohskm X lo-* 1:6 area/volume ratio 2 X, 2 1:1.5 area/volume ratio “CA - _- _ _r_ /3u rpm = & z L3u rpm

METALFINISHING

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DECEMBER1995

Cal/g mol-“K

The values of AH for each parameter with different temperature intervals are shown in Table III. They behave like the activation energies in that the positive and negative signs are at the same operating conditions.

T - Absolute temperature, “K lated from the slope of log K vs. l/T using the method of least squares and depicted in Table III. Based on the activation energy results at different parameter ranges (Table III), it can be concluded that (1) The activation energies have positive values in the temperature interval 3s WC, which renresents th_e ontimum ______ --~--------r --------interval condition because it shows the most suitable and effective one in this chromating process. (2) The activation energy in the temperature range 4050°C has negative values and shows an insignificant effect on the rate of reaction, but it may also have a bad effect on the properties of the chromating layer, so it was disregarded. (3) According to Habashi,’ the activation energy of a diffusion-controlled process is characterized as being 1 to 3 Kcal/g mol, while for chemically controlled processes it is usually greater than 10 KcaVg mol. Thus, it is likely that the mechanism of this chromating process is diffusion controlled. For improving the Arrhenius equation, more elaborate expressions have been derived. The general approach for studies of heterogenous reactions is the consideration of the free ener y change instead of activation energy.! J Hence, a modified Arrhenius equation was established: K = A’ TeWAHRT

CONCLUSIONS The following conclusions may be drawn from the date obtained: 1. From the practical viewpoint, it can be said that the chromating bath is very stable and without sludge at all temperature intervals. This enables the frequent use of the bath, with little replenishment of the consumed species and without f&ration. 2. The temperature has a positive influence on the coating weight (chromating rate) in the temperature interval ‘ W-AWP gnrl D n,an&~~n d” N ” LU‘ U u urgU&” I ,-,n~ “1.l in 111 tha U&l temperature interval 40-50°C for all parameters under investigation. 3. Based on the developed empirical correlations, it is evident that the chromating rate is utmost dependent upon reaction time; more dependent upon potassium fluoride concentration, bath conductivity, and sodium dichromate concentration, respectively; less dependent upon agitation speed; and independent of the surface area to volume ratio.

4. Activation-energy values indicate that the rate-determining step is of a diffusion-type in spite of less dependency of the chromating process upon agitation. This result is valid for every parameter. 5. The negative values of the reaction orders, activation energy, and free

(8)

The free energy AH is computed from the slope of the log plot of K/T vs. l/T.

energy can be neglected because their operating parameter ranges not only have an insignificant effect upon the chromating range; but also an unfavorable one upon the properties of the chromating layer. 6. According to the results of activation energy and free energy, it can be said that activation energy and free energy have a good qualitative agreement because they have the same positive and negative character over the same range of the tested operating conditions; in the temperature interval 30WC, the activation energy has nearly double the value of the free energy; and in the temperature interval 40WC, the activation energy and free energy have approximately the same negative values.

References

._. . . i. wolat, G., *Werksto& und Korrosion, 8:486; 1961

2.

Levitina, E.I., Zh. Fiz. Khim., 342075;

3.

Eppensteiner, F.W. and M.R. Jenkins,

1960 Metal Finishing Guidebook and Directory Issue, 93( lA):468; 1995 4. Federated Republic of Germany, Tentative Standard DIN 50941, Korrosionsshutz, Chromatieren von Zink, Kadmium und deren Legierung, Richtlinien; 1968 El-Mallah, A.T. et al., Metal Finishing, 86( 1):67; 1988 Habashi, F., Principies of Extractive Metallurgy, Gordon and Breach, New

York; 1969, pp. 143-164 Levenspiel, O., Chemical Reaction Engineering, John Wiley, New York; 1962, pp. 25-28 Mason, D.M. and G.M. Simmons, Chemical Engineering Science, 27:75;1 1977 MF

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