The corrosion inhibition of magnetic iron powders by triazine dithiols

Corrosion Science, Vol. 33, No. 6, pp. 831-841h 1992 Printed in Great Britain.

THE

CORROSION POWDERS

(1010-938X/92 $5.011 + 0.0(I © 1992 Pergamon Press pie

INHIBITION OF MAGNETIC BY TRIAZINE DITHIOLS

IRON

KUNIO MORI,* NAOAKI KUMAGAI,* HIROSHI HORIE,+ MITSURU NAKAMURAt

and

SADATO HIRATSUKA+ * Department of Applied Chemistry and + Department of Metallurgy. Faculty of Engineering, lwate University, Ueda, Morioka 020, Japan

Abstract--The surface treatment of ultra fine magnetic iron powders with 6-substituted-l,3,5-triazine2,4-dithiols (RTD) and their corrosion inhibition were investigated. The amount of RTD adsorbed was influenced by trcating time and temperature, solvent, type of RTD and magnetic iron powder. Non-polar solvents and RTD with long alkyl chains in 6-substituted groups provided RTD films with a high layer number on the surface of the powders. Basic and Co-alloyed iron powders also had RTD films with a high layer number in the surface treatment of the powders. They showed lower corrosion rates than acidic magnetic iron powder. Magnetic iron powder changed to a mixture of Fe~.O3 and F e O O H during aging in moist air. RTD treatment was very effective for preventing the corrosion of magnetic iron powder and decreasing its saturated magnetization. The preventive effect increased with RTD film thickness as well as water repellency and insulation. RTD treatment also prevented rapid exothermic reaction of iron powder in air. INTRODUCTION

IN RECENTyears, there has been increasing interest in the use of ultra fine magnetic iron powders in magnetic recording coatings for the production of tape and card recordings with high density. 1-3 Magnetic iron powders are chemically unstable; they readily burn and rust, and do not distribute uniformly in the vehicle. 4 To eliminate these shortcomings, it is necessary to treat magnetic iron powder with organic compounds to import insulating and water-repelling properties, and hydrophobic properties on the surface of magnetic iron powders. A previous paper described that triazine dithiois produce an insulating film on the surface of copper powders and metals, thus inhibiting corrosion ,5 and that treated metals have a strong affinity for polyethylene and other polymers. 6 The surface treated metals with triazine dithiols may effectively inhibit corrosion and have an affinity for polymers. This paper reports the surface treatment of magnetic iron powders by triazine dithiols and inhibition of the powders. EXPERIMENTAL

METHOD

Reagents and materials The magnetic iron powders used were commercially obtained from Toda Kogyo Co. Ltd and Kanto Denka Kogyo Co. Ltd. The details are shown in Table 1. Powders A and D were covered with inorganic films but not oxide film on the surface. Powder C is a Co alloy (Co : 5.1%). Powder B does not contain any film except a thin oxide film such as Fe304 and FeeO 3 on the surface. Powders A, C and D are handled in dry air while Powder B is maintained in benzene or under a vacuum. The abbreviations and chemical structures of 6-substituted groups-l,3,5-triazine dithiols (triazine

Manuscript received 15 July 1991. 831

K. MORI et al.

832 TABLE 1.

PROPERTIESOF MAGNETICIRONPOWDERS

Iron powders

Hc* (Oe)

a~t (emu g - l)

arias

pH

SA:~ (m 2 g - I)

Powder A Powder B Powder C Powder D

1538 1465 1500 1457

122 122 101 123

0.51 0.52 -0.47

6.1 -8.0 10.2

53 52 56 53

*Coercive force. Specific magnetic moment. $ Surface area.

dithiols) used as surface treating agents of magnetic iron powders are listed in Table 2. DB in Table 2 was supplied from Sankyo Kasei Co. Ltd and the others were prepared as previously described.5'7

Surface treatment An organic solution of triazine dithiols, on which de-aeration under vacuum and nitrogen introduction had been conducted three times, was introduced into three-necked flasks (300 ml) each equipped with a nitrogen insert tube. The solution (200 ml), containing triazine dithiols (10 -3 or 10-2 mol 1-1), was elevated to a certain temperature that was maintained constant followed by the introduction of nitrogen gas though the inert tube. Iron powders (1.0 or 10 g) were rapidly poured into the solution and the mixture was stirred under constant conditions. Surface treatment was terminated by filtering the mixture. The film weight of triazine dithiols on the surface of the magnetic iron powders was determined from the concentrations of triazine dithiols in the solution before and after surface treatment by absorptiometry. Corrosion testing Magnetic iron powder (1.0g) was placed in a petri dish (diameter 5 cm) which then was put in thermohydrostat (Toyo Seisakusho Attempter) maintained at constant temperature and humidity. After the corrosion test, the powder was dried under vacuum at 50°C for 60 min in the presence of P205 and used for determination of oxidized iron powder content. Measurements Weight gain, the difference between the weight of iron powder before and after the corrosion test was the weight of oxygen which reacted with the surface of the powder. X-Ray diffraction patterns were obtained with a X-ray diffractometer (Rigaku Denki 5051-AII,CE;Fe-K,). Differential thermal analysis was conducted with a differential thermometer (Rigaku Denki). Saturated magnetization (a~) was measured with a vibration-type magnetometer (Toei Kogyo VSM-3).

TABLE 2. ABBREVIATIONAND CHEMICAL STRUCTUREOFTRIAZINEDITHIOLS(RTD)

R HS~N~ILSH RTD

Abbr.

R--

DM DB DO DL

(CH3)2N-(C4H9)zN-(CsHI7)2N-(CI2H25)2N--

Corrosion inhibition of magnetic iron powders

833

EXPERIMENTAL RESULTS AND DISCUSSION Surface treatment Triazine dithiols are very effective inhibitors and good for the surface treatment of copper and its alloys 7"s but ineffective for the surface treatment of iron and iron alloy plates. 9 A powder is generally considerably more chemically active than a plate or block. Ultra fine magnetic iron powders appear to react with triazine dithiols to produce a film on the surface. Figure 1 shows the effects of treatment time and temperature on the amount of D O adsorbed on magnetic iron powder B. This amount increased with temperature but its influence was very small and increased with treatment time for about 10 rain and then reached a plateau. The rate of film growth followed a parabolic curve as was also noted for copper powder and plates. 5 The rate determining step is thus the diffusion of triazine dithiols. A plateau means that film growth stops under a certain condition. The effects of solvents on the adsorption of triazine dithiols on magnetic iron powders were investigated. As shown in Fig. 2, the amount of D O adsorbed was dependent on the kind of solvent. The relation between this amount and treatment time changed in a similar manner for any solvent but the value of the plateau depended on the solvent: it increased with the decrease in solvent polarity. In hexane, D O gave three layers on the surface of magnetic iron powder although in ethanol it produced only one layer. The number of layers was determined from the surface area occupied by molecules (about 0.30am2), as determined from surface pressure-area isotherms on water, u~ The number of D O layers at each plateau was roughly an integral number such as one, two and three. The films may thus be ordinarily arranged in a cross packed manner on the surface as described previously for surface treatment of copper powder and plates.5,]~ Figure 3 shows the effect of triazine dithiols on the amount of R T D adsorbed on magnetic iron powder B. The rate of film growth increased with the length of alkyl chain of 6-substituted groups in triazine dithiols. The adsorbed weight at each

6

4

'o o

2

o c~

D L

I

I

I

5

10

15

20

Time

(min)

Fro. 1. The effects of treating time and temperature on the amount of DO adsorbed on magnetic iron powder B in the benzene solution: (Q) 20°C; (C)) 35°C; (O) 50°C.

834

K. MORI et al.

8

O---

3

U exane

~o

,-4

0 0

FIG. 2.

5

10

15

Time

(min)

20

The effect of solvents on the a m o u n t of D O adsorbed on magnetic iron powder B in the benzene solution at 40°C.

plateau also increased with the length. Deposition would thus appear to occur according to the diffusion mechanism described previously. 6 Various magnetic iron powders, differing in surface properties, have been shown to improve the corrosion resistance and the dispersion of iron powder. 4 As shown in Fig. 4, the amount of DO adsorbed was dependent on the kind of powder. Powder A, which is acidic, had the lowest rate of film growth and the lowest adsorbed weight at the plateau. Powder D, which is basic, had a higher rate and adsorbed weight than

~4

4

c~ 2

0 0

I

1

I

I

5

10

15

20

Time

FIo. 3.

(min)

The effect of triazine dithiols ( R T D ) on the a m o u n t of R T D adsorbed on magnetic iron powder B in the benzene solution at 40°C.

Corrosion inhibition of magnetic iron powders

835

31 Pow v I~A_.

~

---e)-

o oBz I

[

I

I

[

0

5

10

15

20

Time Fl~. 4.

(min)

The cffect of magnetic iron powders on the amount of DO adsorbed in the ethanol solution al 50°C.

Powder B untreated and Powder A, the reason being due to the triazine dithiols being acidic compounds. Powder C had the highest rate and adsorbed weight, being a Co-adhering Fe alloy. Corrosion and inhibition

Ultra fine magnetic iron powders are easily corroded in moist a i r ) The corrosion of various magnetic iron powders in moist air at 70°C and 90% RH was investigated. The weight gain of powder was measured at various times. The weight gain indicates the amount of oxygen combined with iron powders during a corrosion testing. As shown in Fig. 5, this gain (Y) increased linearly with the root of aging time (t), as follows,5 y = k t °.5

where k is the rate constant of powder corrosion. Table 3 shows the rate constant of corrosion in untreated and DO treated magnetic iron powders. The rate constant of corrosion increased in the order, Powder C < Powder D < Powder B < Powder A. Alloyed and basic magnetic iron powders were hard to corrode in the moist air. Powders A and B, acidic oi neutral, corroded easily. DO treated iron powders exhibited a decrease in the rate constant of corrosion more than untreated powders. Triazine dithiol treatment was quite effective for inhibiting the corrosion of magnetic iron powders. To determine the corrosion products of untreated and D O treated magnetic iron powder B, X-ray diffraction patterns of the powders after being aged were measured, In Fig. 6, the peak at 20 = 52 is assigned to iron. Sample A was oxidized in dry air at 200°C. For this sample, the peak at 20 = 41 was assigned to magnetic F%O 4, indicating corrosion. Samples B and C were aged for 10 days in moist air at 70°C and 90% RH. For these samples, the broad peaks were assigned to a mixture of a-Fe203 and a - F e O O H , indicating wet corrosion. DO treatment of magnetic iron powders retarded the formation of a-F%O3 and (,-FeOOH. The peak

836

K. MORt et al.

IO0

80

60

40

20

0 0

I

I

I

1

2

3

t o . 5 (min

o. s )

FIG. 5. The relationship between weight gain and t °5 in the corrosion of various magnetic iron powders at 70°C and 90% RH: (@) Powder A; ((3) Powder B; ( 0 ) Powder D; (Q) Powder C.

at 20 = 77 based on the (200) surface became broad after aging. This shows corrosion to start from the (200) surface of magnetic iron to produce a mixture of a-Fe203 and a -F eOOH on the surface. Triazine dithiol treatment thus retards the formation of a-Fe203 and a- F eO O H on the surface.

The effects of film thickhess and properties The corrosion of iron powders in moist air can generally be expressed as follows. Anode reaction; Cathode reaction;

Fe ~ Fe + 2e 1/202 + H20 + 2e--~ 2 O H - .

Corrosion reactions of iron powders occur by transfer of electrons and H20 and O2 molecules in interfacial layers of iron powders. This transfer depends on film thickness and properties. Figure 7 shows the effects of the amount of DO film on the corrosion rate. This TABLE 3.

THE

CORROSION RATE

OF

VARIOUS

MAGNETIC IRON POWDERS AT 7 0 ° C AND 9 0 % R H

Corrosion rate (mg g-I t0s) Powders

Untreated

DO-treated*

Powder Powder Powder Powder

38.9 34.8 13.3 20.5

16.9 15.4 5.2 9.5

A B C D

*DO film; 20 mg g - i powder.

Corrosion inhibition of magnetic iron powders

Blank

3~

837

~k_

A

A

B

A

A

.,,~,, I

I

1

I

I

I

I

I

30

40

50

60

70

80

90

100

20 Fro. 6. X-Ray diffraction patterns of magnetic iron powder aged at 70°C and 90% RH: A--Oxidation at 200°C in dry air: B--Untreated powder B, aged for 10 days; C - - D O treated powder B, aged for 10 days.

rate decreased with increase in layer number. An inhibiting effect was evident even when there was less than one layer, since the active points on the surface for corrosion are covered by D O film and electrochemical reactions are retarded. An increase in layer n u m b e r in the D O film quite effectively inhibited the corrosion of

1 O0

-

O g

a0

~ c

o 60

.,4

g " 3~

40

0 0

@

®

2O

0 0

1

2

to

~ (min o

3 ~. )

FIG. 7. TheeffectoftheamountofDOfilmontheweightgaininthecorrosionofmagnetic iron powders at 70°C and 9()% RH: (O) Blank; (@) DO 0.l layer: (O) DO 0.5 layer: (@) DO 1.0 layer; (O) DO 2.8 layer.

838

K. MOR! et al.

100g ~

anck~

8o DM

~

BI

C 60 "~m -

DB

40

20

0

0

I to

FIG. 8.

2 ~ (min

o

3

s )

T h e effect of triazine dithiol films on the weight gain in the corrosion of magnetic iron powders at 70°C and 90% R H . Film weight: 40 ± 5 mg g - l powder.

iron powder by preventing the transfer of electrons and H20 and 02 molecules on the interface. Figure 8 shows the effects of triazine dithiols on corrosion rate. Since corrosion reactions on the surface were prevented by the isolating effects of organic film, the corrosion rate decreased with increase in alkyl chain length of 6-substituted groups and activation energy increased. The isolating effects in corrosion are dependent on water repellency and the insulation of films as well as thickness. The water repellency and insulation of films increase with alkyl chain length of 6substituted groups. By inhibiting the corrosion of magnetic iron powders, magnetic properties can be maintained for a long time. The relation between aging time and the magnetic properties of iron powders was thus investigated. Figure 9 shows the effects of aging time and triazine dithiols on the retention of saturated magnetization (os) of untreated and treated magnetic iron powder D. The saturated magnetization of iron powders decreased with aging time since high magnetic iron changes to low magnetic and non-magnetic iron compounds as a result of corrosion in moist air. Triazine dithiol films on magnetic iron powders prevented a decrease in saturated magnetization since corrosion is retarded. The preventive effect of saturated magnetization increased in the order, DB < DO < DL in which water repellency and insulation of films increase. Another reason for inhibiting corrosion is safety during conveyance and storage of a large quantity of dry magnetic iron powder. Figure 10 shows the exothermal behavior of untreated and treated magnetic iron powders. Untreated iron powder started to generate heat at over 40°C. This generation rapidly increased at somewhat beyond 100°C and then reached a plateau at over 150°C. DO treated iron powder with one layer of film started to generate heat at 90°C. An increase in the number of DO layers retarded the generation of heat. Thus possibly DO treatment decreases the danger of rapidly generated heat during conveyance and storage.

D~

Corrosion inhibitionof magnetic iron powders

I00

u~ 0

839

90

4~ e'

@ 4~

re

80

I 0

100

I

I

I

200

300

400

Aging time (hr) F1¢3. 9.

The change of the saturated magnetization (os) of R T D treated powder B in the aging at 60°C and 80% R H . Film weight: 4(I ± 5 mg g-i powder.

CONCLUSIONS

Surface treatment and inhibition of ultra fine magnetic iron powders were investigated. Magnetic iron powder was treated by various triazine dithiols in solvents. The amount of triazine dithiol adsorbed was dependent on treatment time and temperature, solvent, triazine dithiol, and magnetic iron powder. The corrosion of magnetic iron powder followed a parabolic curve. The surface properties of iron powders affected corrosion rate. Triazine dithiol treatment prevented the corrosion of magnetic iron powders in moist air. The preventive effect of triazine dithiol treatment increased with film thickness and depended on water repellency and insulation. Decrease in saturated magnetization of iron powder during aging in moist air was prevented by treatment with triazine dithiols.

I 50

FIG. 10.

I 100

I

1

I

150

200

250

Temperature ( ° C ) The exothermic behavior of DO treated magnetic iron powders: ( (. . . . . . . .

) D O 0.1 layer; ( - - - - - - ) D O 0.5 layer. ( - - . - - )

1.0 layer.

) Blank:

840

1. 2. 3. 4. 5. 6,

7. 8. 9. 10. 11.

K. MORI et al. REFERENCES Y. TOKUOKAand Y. KAWAKAMI,Kogyo Zairyo 34, 135 (1980). T. IZUMI,Jitsumu Hyoumen Gijutu 32,589 (1985). K. SUMIVAand T. MATSUMOTO,Shikizai 53,211 (1985). S. TOCHIHARA,Progress in Org. Coating 10, 195 (1982). K. MORI, Y. OKAI, H. HORIEand H. YAMADA,Corros. Sci. 32, 1237 (1991). K. MURAKAMI,K. MORIet al., New composite techniques using inorganic and polymeric materials. K. Morn and M. OKUMURA,Nippon Kagaku Kaishi 786 (1977). K. MORI and M. OKUMURA,Nippon Kagaku Kaishi 1477 (1979). K. MORI, Jitsumu Hyomen Gifutsu 35,200 (1988). K. MORI and S. SAI, PolymerJ. 23, 1019-1023 (1991). K. MORI and Y. MUROl,J. Polymer Sci. Part A 24, 2893 (1987).