Journal of Minerals & Materials Characteri zation & Engineering, Vol. 8, No.7, pp 501-511, 2009 Printed in the USA. All rights reserved
Effect of Ch romium on the Corrosion Behavior of Powder-Processed
Fe-0.35 wt % P Alloys
Yashwant Mehta*, Shefali Trivedi, K. Chandra, P.S. Mishra
Indian Institute of Technology, Roorkee- 247667, Uttarakhand, India
*Corresponding Author:
The corrosion behaviour of phospho ric irons (i.e. Fe-P alloys conta ining low phosphorous in th e
range 0.1 to 0.7 wt. %) with/without addition of chromium, prepared by powder forging route
was studied in different environments. The various environments chosen were acidic (0.25 M
H2SO4 solution of pH 0.6), neutral/marine (3.5 % NaCl solution of pH 6.8) and alkaline (0.5M
Na2CO3 + 1.0 M NaHCO3 solution of pH 9.4). The corrosion studies were conducted using Tafel
Extrapolation and Linear Polarization techniques. The studies compare electrolytic Armco iron
with phosphoric irons. It was observed that, chromium improved the resistance to corrosion in
all the environments. Corrosion rates were higher in acid medium due to the enhanced
hydrogen evolution and hence, the cathodic reaction. The corrosion rates were minimal in
alkaline medium and low in neutral solution.
Key words: Phosphoric Iron, Corrosion, Powder Metallurgy, Forged, Ancient iron.
Corrosion of iron or steel is affected by exposed environment [1]. Corrosion depends on both,
the composition of the metal/alloy and the environmental conditions. Important electrolyte
variables affecting corrosion of iron are pH, concentration, fluid flow, temperature and oxidizing
power of the solution [2].The diffusion controlled oxygen reduction predominates in weak acid
and neutral solutions. The corrosion rate is dependent on hydrogen ion concentration in case of
acidic solutions [2].
Further, it is reported that plain carbon steel is subjected to more severe corrosion attack in a
marine environment than in urban and rural media [3]. Segregation of phosphorus to grain
boundaries can strongly affect intergranular stress corrosion cracking (SCC) of irons [4] and
steels [5] in carbonate/bicarbonate solutions. Phosphorus additions are deleterious in its effect on
alloy corrosion resistance and chromium did not affect the behaviour in de-aerated 0.1 N
502 Yashwant Mehta, Shefali Trivedi, K. Chandra, P.S. Mishra Vol.8, No.7
sulphuric acid [6]. The addition of Chromium increased the atmospheric corrosion resistance in
all the cases [7].
In contrast to carbon steel, ancient phosphoric irons that were used for constructing large beams
and located at Konark and Puri in India, have revealed excellent atmospheric corrosion resistance
in saline seashore environments for several hundred years. The 1600-year-old Delhi Iron Pillar is
a living testimony to the remarkable corrosion resistance of phosphoric irons. The presence of
relatively high phosphorus 0.25 wt-% in the Pillar plays a major role in its excellent corrosion
resistance by facilitating the formation of a protective passive film on the surface [8, 9]. Cr and
Cu containing low alloy steel will be suitable for application in an acidic environment while
alloying steel with phosphorus is not beneficial in such an environment [10].
An attempt therefore needs to be made to understand the role of chromium in carbon-free
phosphoric irons. In this paper, the corrosion behavior of two P/M phosphoric irons with 0.00 wt
% C alloyed with/without chromium has been investigated in three different solutions.
For making iron–phosphorous and iron–phosphorous–chromium alloys by powder metallurgical
technique, iron powder (Fe-99.99 wt%, C-0.00 wt%; 200 mesh) was mixed with iron-
phosphide (C-0.00 wt %; 200 mesh), low carbon ferro-chromium (C < 0.01 wt %; 200 mesh).
Whereas, iron and low carbon ferro-chromium powders were of commercial purity, iron-
phosphide powder was prepared by mixing iron powders with ortho-phosphoric acid.
Subsequently reducing heat treatment (800 C/2 h/H2) was given to obtain iron phosphide. The
reactions are as follows:
Fe + H3PO4 = Fe3(PO4)2 (1)
Fe3(PO4)2 +8H2 = Fe3P + 8H2O (2)
The powder blends were manually mixed to make different alloys. About 500 g of each blended
powder mixture was then poured in a mild steel capsule (as shown in Fig. 1).
Fig.1: Cross-section of mild steel capsule used in the present investigation.
Vol.8, No.7 Effect of Chromium on the Corrosion Behavior 503
Subsequently, the encapsulated powders were heated in a tubular furnace at 1150 C for 45 min
in dry hydrogen atmosphere in order to remove the oxide layer from the surfaces of the powders.
Heated capsules were then forged with a 200T capacity friction screw press to make slabs using
a channel die. Two powder metallurgical alloys were made in the present investigation (Table 1).
The compositions of these alloys are based on the powder mixture. Fig. 2 schematically
illustrates the process of making slabs by hot powder forging technique.
Table 1: Composition of phosphoric irons developed in the study & Armco iron
Sample P (wt %) Cr (wt %) Fe
1 (Armco) - - >99%
2 0.35 - Balance
3 0.35 2 Balance
Fig.2: Schematic diagram illustrating the production of slab by hot forging of encapsulated
powder mixture.
504 Yashwant Mehta, Shefali Trivedi, K. Chandra, P.S. Mishra Vol.8, No.7
The slabs were then homogenized at 1200 C for 2 h to eliminate compositional in-homogeneity.
All the alloying elements are present in the form of fine particles around pure iron particles. This
iron particle is 100% gamma-phase at the homogenizing temperature. Phosphorous (in the form
of ferro-phosphorous) combines with this gamma iron powder particle and dissolves in it. As it
dissolves, it gets converted into ferrite (Fig 3, 4) and as ferrite phase grows out of gamma phase,
more and more phosphorous penetrates in it. This helps carry chromium in ferrite phase.
Fig 3: Fe–P binary phase diagram (Kubaschewski 1982).
Fig 4: High temperature gamma loop region of the Fe–P phase diagram (Kubaschewski 1982)
Vol.8, No.7 Effect of Chromium on the Corrosion Behavior 505
The mild steel encapsulation over the slabs was then removed by machining. The slabs, after
removal of mild steel skin, were hot rolled using flat roll at900 C to make thin sheets. Rolling
was carried out very slowly at 900 C with 0.1mm thickness reduction per pass. The rolling
was done using small laboratory scale rolling mill with 10 cm roll diameter. The sheets were
then annealed under vacuum at 950 C for 40 min to relieve the residual stresses. All the samples
prepared this way were characterized in terms of density, microstructure, hardness, and tensile
properties as detailed elsewhere [11].
The samples (15 mm length, 15 mm width, and 2 mm thickness) for electrochemical testing were
cut along the rolling direction. The surfaces were finished using SiC abrasive paper (up to 800
grit) and samples were degreased with acetone. Several samples were prepared from the starting
materials for conducting reproducible experiments. One side of the samples was soldered with a
copper wire. Then the soldered sample was covered with enamel exposing 1 cm2 area. The
samples were mounted in a K0047 Corrosion Cell (used in ASTM standard G-5, supplied by
Ametek, USA) for conducting Tafel polarization studies at a scan rate of 0.166 mV/ s. The Tafel
extrapolation method (conducted as per ASTM Standard G3-89 [12]) was utilized for
determining icorr of the phosphoric irons and Armco electrolytic iron in 3.5 % NaCl having pH
Since the cathodic reaction was primarily diffusion controlled in the case of 3.5 % NaCl solution
having pH 6.8, the activation-controlled anodic Tafel region was extrapolated to intersect the
horizontal drawn at zero current potential to obtain the corrosion rate [13].
Linear Polarization technique was used to evaluate the corrosion rates of the phosphoric irons in
the following solutions: 0.25 M H2SO4 of pH 0.6, 3.5 % NaCl of pH 6.8 and 0.5M Na2CO3 + 1.0
M NaHCO3 solution of pH 9.4. A scan rate of 0.166 mV/s was used. Corrosion rate in
penetration units (like mils/year, mpy), was calculated from icorr using the following equation
  1
Where Λ =1.2866×105 ( (
i = icorr =the corrosion current density in Amps/ cm2 (Amp=1 Coulomb/ s)
ρ =density (7.86 g/ cm3, for iron)
ε=equivalent weight (27.56 g/ equivalent, for iron)
The solutions were prepared using chemicals of analytical grade reagent and single distilled
water. A digital pH meter (Phillips, model 9045) was used for recording pH of the solutions at
room temperature. The pH meter was calibrated using three different standard pH solutions
before recording pH.
An EG&G PARSTAT 273A Potentiostat (Ametek, USA) and a saturated calomel reference
electrode (SCE) were used in all electrochemical experiments. The open circuit potential (OCP)
was stabilized for 1 hr before the start of each experiment. All the experiments were repeated
three times.
506 Yashwant Mehta, Shefali Trivedi, K. Chandra, P.S. Mishra Vol.8, No.7
The corrosion rates determined by the Tafel extrapolation method in aerated solution of 0.25 M
H2SO4 of pH 0.6 & 3.5 % NaCl of pH 6.8 and those determined by the linear polarization
method in aerated solutions of 0.5M Na2CO3 + 1.0 M NaHCO3 of pH 9.4 are discussed
separately below. The cathodic reaction for the samples obtained in all the three solutions
discussed above consists of a composite reaction of [15] hydrogen evolution
2H++2eH2, 2H2O+2e-H2+2OH
And oxygen reduction
This is also evident from Pourbaix diagram of the Fe-H2O system.
3.1. Tafel Extrapolation Method
3.1.1. 0.25 M H2SO4 solution (pH 0.6)
As pH decreases, hydrogen evolution rate will dominate over oxygen reduction rate at Ecorr.
Thus, at pH 0.6, the contribution of hydrogen evolution at Ecorr is significant. The exchange
current density io for hydrogen evolution H+/H2 and icorr increase on increasing the concentration
of H+ ion or decreasing the pH [16]. Hence corrosion rate increases.
The beta anodic & cathodic slopes, Ecorr, icorr and corrosion rate (mpy) obtained from the tafel
curves of the samples are tabulated in Table 2 and displayed in Fig. 5. The Ecorr lies between -466
mVsce & -523 mVsce. The corrosion rate for sample 3 is 61 mpy. This is similar to that obtained
for copper-chromium based corrosion resistant TATA steel (46 mpy). The corrosion rate for
sample 2 is 185 mpy and is much less than that obtained for plain carbon steel (250 mpy) [10].
-6 -5 -4 -3 -2 -1
log i (Amp/cm2)
Esce (mV)
Figure 5: Tafel curves for Phosphoric irons in 0.25 M H2SO4 solution (0.6 pH).
Vol.8, No.7 Effect of Chromium on the Corrosion Behavior 507
Table 2: Corrosion data (Tafel) for Phosphoric irons in 0.25 M H2SO4 solution (0.6 pH)
Sample Beta Anodi
V/decade Beta Cathodi
V/decade E(I=0)
mV Icorr
µA Corrosion Ra
1 -509.0E33 181.9E-3 -553.0 174.7 78.8
2 29.69E-3 111.6E-3 -466.0 410.1 185.0
3 75.04E-3 138.3E-3 -523.6 135.1 60.9
3.3.2. 3.5 % NaCl solution (pH 6.8)
The Tafel plots obtained in aerated 3.5 % NaCl solution are shown in Fig. 6. Tafel polarization
curves showed diffusion controlled cathodic reaction in all cases. In this case, the diffusion-
controlled oxygen reduction reaction is the dominant reaction at Ecorr [15].The activation
controlled anodic Tafel slopes, icorr and corrosion rate (mpy) obtained from the Tafel polarization
curves of samples (Fig. 5) are tabulated in Table 3. Since the cathodic reaction is diffusion
controlled, hence the cathodic Tafel slopes are not provided in Table 3 [13].
-8 -6 -4 -2
log i (Amp/cm2)
Figure 6: Tafel curves for Phosphoric irons in 3.5 % NaCl solution (6.8 pH)
Table 3: Corrosion data (Tafel) for Phosphoric irons in 3.5 % NaCl solution (6.8 pH).
Beta Anod
mV Icorr
µA Corrosion Ra
1. 67.08E-3-424.6 2.897 1.3
2. 61.15E-3-559.4 16.42 7.4
3. 37.20E-3-479.8 7.644 3.4
508 Yashwant Mehta, Shefali Trivedi, K. Chandra, P.S. Mishra Vol.8, No.7
The corrosion rate was obtained from the Tafel extrapolation method matched with literature
data, as discussed below. The corrosion rate of iron is 12 mpy as obtained by the Tafel
extrapolation method after 24 hours of immersion in unstirred, air saturated 3.5 % NaCl solution
[17]. The actual corrosion rate of plain carbon steel (AISI 1020 steel) in quiet surface water is up
to 15 mpy in the first year and then decreases to 5 mpy after 1000 days [18]. The Ecorr of the
samples lies between 600 mVSCE and 690 mVSCE . The corrosion rate of samples obtained by
the Tafel extrapolation method after 1 h immersion in 3.5 % NaCl was in the range of 6 to 10
mpy [10].
In the present study, the corrosion rate of samples obtained by the Tafel extrapolation method
after 1 h immersion in 3.5 % NaCl was in the range of 3.4 to 7.4 mpy. The Ecorr of the samples
lies between 480 mVSCE and 560 mVSCE.
3.2. Linear Polarization Method
3.2.1. 0.5M Na2CO3 + 1.0 M NaHCO3 solution (pH 9.4)
This solution was chosen to evaluate the corrosion resistance of the samples against soil (buried
condition). Alkaline solutions are known to cause intergranular SCC due to segregation of
Phosphorous in low alloy steels [19].
The Rp, Ecorr, icorr and corrosion rate (mpy) obtained from the linear polarization curves of
samples are tabulated in Table 4 and displayed in Fig. 7. The Ecorr lies between -241 mVsce & -
243 mVsce. The corrosion rate is low and lies between 0.46-0.57 mpy (mils per year). These
materials can be used in buried (in soil) conditions.
Fig 7: Bar chart displaying corrosion data (Linear Polarization) for Phosphoric irons in 0.5M
Na2CO3 + 1.0 M NaHCO3 solution (9.4 pH)
Vol.8, No.7 Effect of Chromium on the Corrosion Behavior 509
Table 4: Corrosion data (Linear Polarization) for Phosphoric irons in 0.5M Na2CO3 + 1.0 M
NaHCO3 solution (9.4 pH).
3.3. Segregation of Alloying Elements
Powder metallurgical phosphoric irons developed in the present investigation are free of any
segregation of the alloying elements along the grain boundaries. They get distributed uniformly
in the entire structure. This has been confirmed by:
(i) Optical Microscope (Fig 8),
(ii) Surface Morphology (SEM) and EDAX Pattern from different Spots (Fig 9, 10),
(iii) Composition Image [Secondary Image] & X- Ray Mapping (Fig 11).
The microstructures show that most of the pores are away from the grain boundaries. This is
good for the mechanical properties as well as resistance to corrosion. Furthermore, residual
porosity in these samples (of the order of 2 vol %) has no adverse effect under moderate
corrosion conditions as described above.
Fig 8: Microstructures of rolled and annealed alloys etched with 2% Nital.
Pores are elongated in the rolling direction. (a)Sample 2 & (b) Sample 3.
Ohms E(I=0)
mV Icorr
µA Corrosion Rat
1. 89600 -252 0.2424 0.11
2. 17170 -242.4 1.265 0.57
3. 21350 -241.1 1.017 0.46
510 Yashwant Mehta, Shefali Trivedi, K. Chandra, P.S. Mishra Vol.8, No.7
Fig 11: Composition Image [Secondary Image] & X- Ray Mapping of Sample 3.
1. The compositions containing chromium exhibited lower rate of corrosion than the
composition containing Fe-P only.
2. These materials are suitable for marine and alkaline conditions.
3. The composition containing 0.35 P and 2 Cr corrodes at a rate lower than that of Armco
iron in H2SO4 solution.
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