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PHOSPHATATION OF Cu-CONTAINING IRON POWDER METALLURGICAL SAMPLES FOR CORROSION

PROTECTION

H. ASHASSI-SORKHABI*, D. SEIFZADEH, E. ASGHARI*

*Physical Chemistry Department, Faculty of Chemistry, University of Tabriz, Tabriz, Iran

ABSTRACT

Phosphatation of Cu-containing powder metallurgical samples were made in different bath compositions. Measurement of coatings weight showed that the results from two baths (same components with different ratio of H3PO4 and ZnO) didn't change significantly in successive phosphating of samples, but in the case of bath (same components with some additives), coatings weight reduced remarkably. Corrosion rate of samples decreased significantly after phosphating. Zn and P peaks in EDX spectra confirmed phosphate layers formation. SEM image of the sample phosphated in the bath without additives showed that phosphate layer is amorphous whereas for samples obtained from other baths were amorphous-crystalline.

1. INTRODUCTION Metals have been the backbone of civilization. Efforts have been spared to find alternatives and replacements for metals but these still play a major role in the manufacture and construction and are likely to do so for many more years. This is due to the combination of several useful properties like strength, workability, low-cost and ability to be recycled, that the metals possess. However, metals which are extracted from their ores by chemical or electrochemical means show a strong tendency to revert to their oxide form at the first available opportunity, i.e., they tend to corrode [1-3] and as a result they create a tremendous economic loss besides posing a serious threat to the national resources of a country.

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The methods of corrosion prevention are many and varied. These methods may be generally classified as [3]:

Modification of the metal by alloying and/or surface modification; Modification of the environment by the use of inhibitors; and Change of metal/environment potential by cathodic or anodic protection.

The most commonly used method of corrosion protection involves bulk alloying or surface modification. Surface modification is however, far more economical than bulk alloying and is more widely practiced. The methods generally used for surface modification involve the formation of a physical barrier to protect the metal against its corrosive environment [4]. This can be achieved by relatively more modern methods such as: (i) physical vapour deposition (PVD); (ii) chemical vapor deposition (CVD); (iii) ion implantation; (iv) laser treatment; (v) deposition by thermal spray, plasma spray and arc methods; (vi) nitriding; (vii) carbiding; etc., or through more conventional techniques such as: (i) painting; (ii) anodizing; and (iii) chemical conversion coatings. Phosphating process can be defined as the treatment of a metal surface so as to give a reasonably hard, electrically non-conducting surface coating of insoluble phosphate which is contiguous and highly adherent to the underlying metal and is considerably more absorptive than the metal. The coating is formed as a result of a topochemical reaction, which causes the surface of the base metal to integrate itself as a part of the corrosion resistant film [5]. All conventional phosphating solutions are dilute phosphoric acid based solutions of one or more alkali metal/heavy metal ions, which essentially contain free phosphoric acid and primary phosphates of the metal ions contained in the bath [6-9]. When a steel panel is introduced into the phosphating solution a topochemical reaction takes place in which the iron dissolution is initiated at the microanodes present on the substrate by the free phosphoric acid present in the bath. Hydrogen evolution occurs at the microcathodic sites:

Fe + 2H3PO4 → Fe (H2PO4)2 + H2↑ (1)

The formation of soluble primary ferrous phosphate leads to a concurrent local depletion of free acid concentration in the solution resulting in a rise in pH at the metal/solution interface. This change in pH alters the hydrolytic equilibrium which exists between the soluble primary phosphates and the insoluble tertiary phosphates of the heavy metal ions present in the phosphating solution, resulting in the rapid conversion and deposition of insoluble heavy metal tertiary phosphates [6-9]. In a zinc phosphating bath these equilibria may be represented as [5]:

Zn(H2PO4) ↔ ZnHPO4 + H3PO4 (2)

3ZnHPO4 ↔ Zn3(PO4)2 + H3PO4 (3)

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A certain amount of free phosphoric acid must be present to repress the hydrolysis and to keep the bath stable for effective deposition of phosphate at the microcathodic sites. Another factor affecting the shift in the primary to tertiary phosphate equilibria is the temperature of the bath. Higher temperatures favor easy precipitation of the tertiary phosphates in a shorter time. Hence, more amount of phosphoric acid is needed for the baths operating at higher temperatures. In contrast, in the case of phosphating baths operated at room temperature, the possibility of the increase in acidity during continuous operation is more likely [10, 11] and is normally neutralized by the addition of the carbonate of the metal which forms the coating (Zn(CO3)2 in zinc phosphating bath). Hence, depending upon the working temperatures and the concentrations of the constituents in the bath, the free phosphoric acid content must be chosen to maintain the equilibrium condition. Too much of phosphoric acid not only delays the formation of the coating, but also leads to excessive metal loss [5].

Powder metallurgy materials have been widely used in airplanes, trains and automobiles. The traditional powder metallurgical materials mainly include Fe-based and Cu- based, which are prepared by pressing and sintering based on metal powders with the addition of lubricants and friction particles. Fe-based materials have high friction coefficient, good heat resistance and low cost. In recent years, the speed and load of engineering machines have increased greatly, which brings forward higher requirements for powder metallurgical [12, 13]. Phosphating is a way to protect metal surfaces from corrosion. The most commonly used phosphate coatings for corrosion protection are based on zinc, iron and manganese [14]. In our previous work [15] phosphating of iron powder metallurgical samples was studied. The effect of phosphating temperature on coatings morphology and corrosion resistance of samples was also investigated. The aim of present work is to study possibility of phosphating on Cu-containing iron powder metallurgical samples, made in Tabriz Powder Metallurgy Company (TPMCO). Reduction of efficiency of each bath is investigated through successive phosphating of several samples and measuring the weight of coatings created on metal surface. Corrosion resistance and corrosion rate of uncoated and coated samples is investigated using potentiodynamic polarization technique. The morphology and composition of coatings is also studied using Scanning Electron Microscopy (SEM) images and EDX spectra. 2. EXPERIMENTAL Cu-containing metallurgical samples, made in Tabriz Powder Metallurgy Company (TPMCO) were used as substrate metal for zinc phosphating. The composition of samples is shown in Table 1. The samples were polished with SiC paper up to 2500 grade, degreased with acetone, immersed in concentrated HCl solution for 30s, rinsed with distilled water and dried using airflow at room temperature. Immediately after pretreatment process the samples were immersed in phosphating bath for 15 minutes,

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the temperature was maintained at 90±1ºC using a thermostat (Memmert, Germany). The volume of each bath was fixed in 100mL and the baths were stirred in constant time intervals during phosphating process in order to reduce the concentration polarization.

Table 1. Composition of Cu-containing iron powder metallurgical samples Fe Ni Cu Mo Mn Si C S P

Bal. 1.75 1.50 0.50 0.15> 0.05> 0.02> 0.015 0.015> Different bath compositions were examined and three optimum bath compositions were used for further tests. Compositions of the selected baths are given in Table 2.

Table 2. The composition of selected baths* Bath No. 1 2 3 H3PO4 45.0 17.5 19.8 HNO3 23.0 ----------- 23.5 ZnO 24.5 3.20 14.7 NaNO3 ----------- 0.17 ----------- NaNO2 ----------- 0.83 ----------- NaF ----------- 1.70 ----------- Di Ethanol Amine ----------- 0.18 ----------- Sodium Tartarat ----------- 2.20 -----------

*All concentrations are in g/L. Reduction of efficiency of each bath was investigated through successive phosphating of several samples and measuring the weight of coatings created on metal surface. In order to determine the corrosion resistance of coated and uncoated powder metallurgical samples the potentiodynamic polarization technique was used in a conventional three electrodes cell configuration connected to an Autolab PGSTAT30 computer controlled potentiostat-galvanostat. The powder metallurgical samples were used as working electrode. A Pt rod and an Ag/AgCl (3M) were selected as counter and reference electrodes respectively. The polarization resistance, Rp, corrosion current density, icorr, and the corrosion rate were obtained from polarization curves in a 3% NaCl solution as aggressive medium. The morphology and composition of coatings were also studied by SEM and EDX analysis. Observations were made before and after immersion of samples in the phosphating baths. 3. RESULTS The phosphating of samples was performed in each bath and the weight of coatings was measured. In order to verify the creation of phosphate coating on powder metallurgical samples the scanning electron microscopy (SEM) images of phosphated samples were

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obtained. Figure1 shows the SEM images and EDX spectra of an uncoated sample and samples phosphated in different used baths. Creation of amorph phosphate layers on surface was confirmed from these images.

(c)

(b)

(a)

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Figure 1. Morphology and EDX analysis of substrate (a) phosphated samples in bath 1 (b) bath 2 (c) and bath 3 (d)

Presence of Zn and P peaks in the EDX spectra of all phosphated samples confirms the phosphate layers formation on substrate. SEM image of the sample phosphated in bath 1 shows that the phosphate layer formed on metal mainly has an amorphous appearance whereas for samples phosphated in bath 2 and 3 the films are formed of both amorphous and crystalline phosphate layers. In industrial applications a phosphating bath is used for several times, thus the efficiency (longevity) of baths is an important industrial factor. In order to investigate the efficiency of baths, successive phosphating of several samples in each bath was carried out. The first sample was immersed in a 100mL phosphating solution for 15 minutes, then the next samples was phosphated in this way and the weight of formed films was determined. Figure 2 shows the weight of coatings created on sample surface during successive phosphating of seven samples.

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.01

1 2 3 4 5 6 70

0.002

0.004

0.006

0.008

0.01

0.012

1 2 3 4 5 6 7

(a) (b)

(d)

Wei

ght o

f pho

spha

te la

yer

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0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.01

1 2 3 4 5 6 7 Figure 2. Variation of coating weight created during successive phosphating of several

samples in bath 1 (a) bath 2 (b) bath 3 (c) It is clear that the weight of coating created on first sample (sample no. 1) in bath 2 is higher than the weight of phosphate layer created on sample 1 in baths 1 and 3. It is also clear from Figure 2 that for samples phosphated in bath 2, a significant decrease in weight of coating is observed that is due to rapid falling of bath efficiency, such an observation can be attributed to the low concentration of bath 2 compounds. In order to study the corrosion properties of phosphated samples the polarization curves were plotted with a scan rate of 2mVs-1. The electrolyte for corrosion studies were 3% NaCl solution and the temperature was maintained at 25 ± 1°C. Figure 3 shows the polarization curves for uncoated and phosphated samples. The polarization resistance, corrosion current density and corrosion rate for each sample were obtained from polarization curves and are given in Table 3. It is clear from these data that phosphating elevates the polarization resistance thus the corrosion current density and corrosion rate decrease. The anti-corrosive properties of phosphate films result from their high electrical resistance. As the charge transfer through a porous film on metal surface is more prevented the corrosion resistance of the metal increases and thus the corrosion rate decreases. Thus, the lower values of corrosion rate after phosphating process can be attributed to the formation of insulator phosphate film on powder metallurgical samples. These insulator films retard the charge transfer between metal and electrolyte. On the other hand, the created phosphate film forms a barrier layer, thus diffusion of aggressive ions from solution toward metal surface and corrosion products, the cations created during metal oxidation, from metal surface toward bulk solution occurs in much slower rates. The anodic and cathodic half- reactions of the corrosion process in NaCl solution are the oxidation of metal and reduction of oxygen. Due to the low conductivity of phosphate layer, the reduction of oxygen is also retarded. Because of all above-mentioned reasons, phosphating cause to enhancement of corrosion resistance and reduction of corrosion rate.

(c) W

eigh

t of p

hosp

hate

laye

r

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1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

1.00E-01

1.00E+00

-1.10 -1.00 -0.90 -0.80 -0.70 -0.60 -0.50 -0.40 -0.30

Figure 3. The polarization curves of uncoated and phosphated samples

Table 3. The corrosion characteristics of uncoated and phosphated samples

Sample Polarization Resistance (Ω.cm2) Corrosion Current Density

( µA/cm2) Corrosion Rate

(mm/yr) Uncoated 532 54.3 0.638

Bath 1 4734 7.60 0.045 Bath 2 1099 40.2 0.236 Bath 3 1043 46.4 0.272

It can be also observed from data presented in Table 3 that corrosion rate of the sample phosphated in bath 1 is much lower than two other phosphated samples. Such an observation can be due to the high concentrations used in bath 1 compared to baths 2 and 3. 4. CONCLUIONS Cu-containing iron powder metallurgical samples, made in Tabriz Powder Metallurgy Company (TPMCO) were phosphated in three different baths at temperature 90±1°C. Formation of zinc phosphate coating on samples was confirmed using SEM images and EDX spectra. The existence of Zn and P peaks in EDX spectra was attributed to the phosphate layer formed on substrate. It was found that the weight of phosphate coating created on first sample in bath 2 is higher than the weight of coatings created in baths 1 and 3. It was also observed that the weight of coatings reduces during successive

log

i (A

.cm

-2)

E(V/Ag-AgCl)

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phosphatings in a bath; in the case of bath 2 this reduction is much more significant than two other ones. It means that bath 2 rapidly loses its efficiency. Corrosion studies on phosphated samples showed that the corrosion rate of Cu-containing iron powder metallurgical samples is strongly reduced after zinc phosphating process. 5. ACKNOWLEDGEMENT The authors thank the Tabriz powder Metallurgy Company for proposing the project, SUPCO (Iran Khodro) for financial support and the Industrial Relations Office of University of Tabriz. 6. REFERENCES [1] Fontana, M.G., Corrosion Engineering, 3rd Edition, McGraw-Hill Book Company,

Singapore, 1987. [2] Evans, U.R., An Introduction to Metallic Corrosion, 3rd Edition, Edward Arnold

Publishers Ltd., London, 1981. [3] Uhlig, H.H., Corrosion and Corrosion Control, 2nd Edition, John Wiley & sons

Inc., New York, 1971. [4] Leidheiser, H., In: Metals Handbook, Vol. 13, 9th Edition, American Society of

Metals, Ohio, 1987. [5] Sankara Narayanan, T.S.N., “Surface Pretreatment by Phosphate Conversion

Coatings-a review”, Reviews on Advanced Materials Science, Vol. 9, pp. 130-177, 2005.

[6] Freeman, D.B., Phosphating and Metal Pretreatment - A Guide to Modern Processes and Practice, Industrial Press Inc., New York, 1986.

[7] Rausch, W., The Phosphating of Metals, Finishing Publications Ltd., London, 1990.

[8] The Pyrene Co. Ltd., “Improvements in or relating to improving the resistance of metals to corrosion”, British Patent 517049, 1940.

[9] Lorin, G., Phosphating of Metals, Finishing Publications Ltd., London, 1974. [10] Sankara Narayanan, T.S.N., Subbaiyan, M., “Critical Evaluation of a Cold

Phosphating Bath”, Plating and Surface Finishing, Vol. 80, pp. 72-74, 1993. [11] Sankara Narayanan, T.S.N., “Performance Evaluation of Phosphating

Formulations in Continuous Operation”, Metal Finishing, Vol. 94, pp. 40-43, 1996.

[12] Li, J., Yin, Y., Ma, H., “Preparation and Properties of Fe3Al-based Friction Materials”, Tribology International, Vol. 38, pp. 159-163, 2005.

[13] Yang, Y.L., “Sintering metal friction materials”, Mechanical Engineering Materials, Vol. 19(6), pp. 18-21, 1995.

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[14] Weng, D., Jokiel, P., Uebleis, A., Boehmi, H., “Corrosion and Protection Characteristics of Zinc and Manganese coatings”, Surface and Coatings Technology, Vol. 88(1), pp. 147-156, 1996.

[15] Ashassi-Sorkhabi, H., Seifzadeh, D., Harrafi, H., “Phosphatation of Iron Powder Metallurgical Samples for Corrosion Protection”, Journal of the Iranian Chemical Society, Vol. 4(1), pp. 72-77, 2007.