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Journal of Minerals & Materials Characterization & Engineering, Vol. 8, No.10, pp.803-811, 2009
jmmce.org Printed in the USA. All rights reserved
Corrosion Behavior of 18/8 Stainless Steel
and Nickel-Plated Low Carbon Steel in Cassava Fluid
O.O. Oluwole1*, P.O. Atanda2, O.A. Odekunbi2 and E. Odegbaju2
1Mechanical Engineering Department,University of Ibadan, Nigeria
2Materials Science and Engineering Department,Obafemi Awolowo University, Nigeria
*Corresponding author: firstname.lastname@example.org
This research work investigated the corrosion resistance of nickel- plated medium carbon steel
and 18/8 stainless steel in cassava fluid (i.e. containing hydrogen cyanide). It simulated the
effect of continuous use of the materials in a cyanide environment where corrosion products are
left in place. Low carbon steel sample was nickel electroplated at 4V for 35 minutes. The plated
sample, the unplated and the 18/8 stainless steel were then subjected to a cassava fluid
environment for thirty days. The electrode potentials, in mV (SCE), were measured every day.
Weight loss was determined at intervals of 5 days for duration of the exposure period. The result
showed little corrosion attack on the nickel-plated steel on the fifth and tenth days which quickly
dropped to zero by the 15th day and remained at the passive state till the 20th day when corrosion
picked up again increasing steadily, linearly until the end of the test day. Corrosion of the 18/8
stainless steel was very low as well decreasing till the last day of the test. The pH of the cassava
solution which initially was acidic because of the cyanide content in the cassava was observed to
progress to neutrality within five days and to alkalinity at the end of the thirty days test (because
of corrosion product contamination of the cyanide). Un-plated steel was found to be unsuitable
for the fabrication of cassava processing machinery because of the very high corrosion rate.
18/8 stainless steel was found suitable for use in this environment. The renewed corrosion
activity on nickel plated steel after the 20th day (pH=12) of continuous use in cyanide
environment makes it unsuitable for use.
Keywords: Corrosion resistance,18/8 stainless steel, nickel-plated low carbon steel, Cassava
804 O.O. Oluwole, P.O. Atanda, O.A. Odekunbi and E. Odegbaju Vol.8, No.10
Corrosion has been established in uncoated mild steel used in machinery for agro-processing1,2.
Previous work on zinc plating on steel used for cocoa and cassava processing machinery showed
that zinc plating did not offer much protection because of the presence of ethanoic acid and
cyanide in the respective fluids during processing3,4.
This work has studied the corrosion behaviour of nickel plated low carbon steel and 18/8
stainless steel in cassava fluid environment.
2. MATERIALS AND METHOD
The materials used for this study were austenitic 18-8 stainless steel and low carbon steel rods.
The low carbon steel was obtained from Universal Steel Company Limited, Lagos, Nigeria while
the 18-8 Stainless Steel was procured from steel vendors. The chemical composition of the low
carbon steel is presented in Table 1.
Table 1. Chemical Composition (wt %) of low carbon steel.
Elements Composition (wt %)
Vol.8, No.10 Corrosion Behavior of 18/8 Stainless Steel 805
2.2.1 Preparation of specimens (surface)
The plated low carbon steel samples were machined into cylindrical pieces of 8 mm diameter
and 20 mm length while the 18/8 Stainless Steel samples were machined into 8 mm diameter and
40 mm length. The samples surfaces were treated by abrading them through successive grades of
silicon carbide papers of grades P60, P120, P320, P400 and P600 grit, and finally on the emery
cloth. They were rinsed in distilled water and then in acetone before drying. The prepared
samples were stored in desiccators until when used for the experiments.
2.2.2 Preparation of cassava fluid
Fresh cassava tubers were procured, grated, and their fluid was manually squeezed out into a
clean bowl and stored in a 10 L container. The chemical composition of the cassava fluid was
carried out at Central Science Laboratory, Obafemi Awolowo University, Ile-ife (OAU). The
result is presented in Table 2.
Table 2. Cassava Fluid Properties.
Composition Composition (average)
Water content, vol.-%93.71
Ash, vol.-% 2.45
Protein, vol.-% 1.06
Lipid, vol.-% 0.17
Carbohydrate, vol.-% 2.80
Acidity, % 1.63
HCN, mg c-1 26.70
2.2.3 Preparation of nickel electroplating bath
Nickel plating solution was prepared in conformity with the Watts Solution in which the
chemical reagents used were nickel sulphate, nickel chloride, boric acid and saccharin as the
806 O.O. Oluwole, P.O. Atanda, O.A. Odekunbi and E. Odegbaju Vol.8, No.10
2.2.4 Samples pretreatment before electroplating operations
The low carbon steel sample was removed from the desiccators in turn and pickled in 0.5M
H2SO4 for 2 min and rinsed in distilled water in order to remove rust. The pickled samples
were degreased in a 100 liter electrolytic degreasing tank containing 200g KOH and 100g NaOH
in distilled water for 2 minutes, after which the samples were rinsed in distilled water. The
samples were weighed using a digital weighing balance model Mettler Toledo Pb153 of ± 0
.001g accuracy and the weight was recorded as the initial weight.
2.2.5 Electroplating operation
The laboratory nickel electroplating bath was stirred with the aid of a stirrer for 1 min. The
sample already attached to the flexible copper wire was then hanged on the cathode arm of the
nickel electroplating bath after which the electroplating rectifier was switched on. The
electroplating rectifier was regulated to 4 V.
Earlier work done on nickel electroplating of manganese steel6 for 35minutes gave good plating
thickness. The steel sample was electroplated for 35mins. The unplated sample was kept as
2.2.6 Corrosion resistance in cassava fluid
The nickel electroplated low carbon steel and the 18/8 stainless steel samples were immersed in
cassava fluid for durations up to 30 days, including an unplated sample as control.
Electrodepotential (mV) measurements between the sample surface and the corrosive
environment were carried out at regular interval of 24 h using a DT8300D digital multimeter
with a zinc electrode used as a reference electrode. The reference electrode was not left in the
cell for the duration of the experiment but used only at time of measurement of potential then
afterwards removed. Values obtained were converted to saturated calomel electrode (SCE)
values with the use of the formula; Ezn-1030mV = S.C.E values7. The corrosion samples were
removed from the corrosion environment with the aid of a tong after which the samples were
properly cleaned in distilled water and dried with cotton wool. The dried samples were weighed
with the digital chemical weighing balance and recorded and this continued at regular intervals
of 5 days. Corrosion rates in mm/yr were obtained from weight loss analysis8 using the
expression CPR=87.6W/ρAT (mm/yr); where W is the weight loss in mg; ρ is the density of
the specimen in g/cm3; A is the total exposed surface area of the specimen in cm2; and T is the
exposure time in hours; mm/yr is the corrosion penetration rate expressed in millimeters
penetration per year.
Vol.8, No.10 Corrosion Behavior of 18/8 Stainless Steel 807
Table 1 shows the nominal chemical composition of the steel samples used while Table 2 shows
the average properties of cassava fluid. The electroplating mass gains, and corresponding coating
thicknesses value, for the zinc plated steel sample is shown in Table 3. Figure 1 shows the
variation in pH values for cassava fluid during the corrosion test period. It shows the pH of the
corrosive fluid changing from acidic to neutrality and then to alkalinity at the end of the
Table 3. Weight Deposited (g) of Nickel on low carbon steel at 4Volt and
35 min Plating Time
per unit area
0.0045 0.4 0.5
0510 1520 2530 35
Exposure Time (Days)
pH(Cassava Fluid with18-8
pH(Cassava Fluid with Nickel
pH (Cassava Fluid with Unplated
Fig. 1. Plot of pH values of Cassava Fluid during Corrosion of 18-8 Stainless Steel and the Low
808 O.O. Oluwole, P.O. Atanda, O.A. Odekunbi and E. Odegbaju Vol.8, No.10
Figure 2 shows the variation in the electrode potential in mV obtained for 18/8 stainless steel,
un-plated and nickel plated low carbon steel samples at various electroplating times.
For the unplated sample, the electrode potential increased from the fifth day until the thirteenth
day after which it started decreasing again and relatively rapidly moved to values consistent with
the exposure of steel. The nickel-plated steel started with a slightly higher potential than the
unplated steel but followed the same trend as the unplated steel till the thirteenth day when it
decreased into passivity from the fifteenth to the twentieth day.
After the 20th day, the potential started increasing again showing renewed corrosion activity.
18/8 stainless steel showed a cyclical trend of increase and decrease in potential from the first
day till the 21st day when a steady plateau was reached till the 30th day.
0510 15 20 25 30 35
Elec trod e Potential mV(SCE)
Nickel plated Steel
Unplated steel sample
18-8 Stainless Steel
Fig. 2. Plot of Electrode potential Versus Exposure Time for 18/8 Stainless Steel and
unplated and nickel plated low carbon steel samples
Figure 3 shows the variation of corrosion rate in mm/yr for 18/8 stainless steel, the nickel plated
low carbon steel sample as well as the corrosion rate of the un-plated sample immersed in
cassava fluid. It shows a relatively low corrosion rate experienced by the nickel plated steel in
the first 10 days of immersion compared to the corrosion rate of the unplated steel. Corrosion
rate was observed to drop to zero on the 15th day and remained zero till the 20th day when a
renewed corrosion activity commenced and increased linearly till the end of the test. The 18/8
Vol.8, No.10 Corrosion Behavior of 18/8 Stainless Steel 809
stainless steel had relatively very low corrosion rate compared to the unplated steel in the first
five days. The corrosion rate continued to drop till the end of the test.
0510 15 20 25 30 35
Exposure Time (Days)
Corrosion Rate (mm/yr)
18/8 Stainless Steel
Nickel Plated Low Carbon
Unplated Low Carbon Steel
Fig. 3. Superimposed Plot of corrosion rate Versus Exposure Time for 18-8 Stainless
Steel, Nickel Plated Low-Carbon Steel And the Unplated Low Carbon Steel
3.1.1 Effect of exposure time on potential
The un-plated sample showed potential increase from the fifth day till the thirteenth day after
which it started decreasing again and relatively rapidly moved to values consistent with the
exposure of bare steel (-550mV, SCE). The pH change during the immersion was significant. By
the fourth day, pH had become neutral and on the 8th day was already alkaline. This affected the
potential of the sample in the corrosion environment. Visible corrosion product for steel (i.e.
rust) was apparent from early immersion resulting in the changing pH values.
The plated sample’s trend of increasing potential from the fifth day till the thirteenth day when it
decreased into passivity till the twentieth day is traceable to the changing pH of the corrosion
environment as well. After the 20th day, the potential started increasing again showing renewed
810 O.O. Oluwole, P.O. Atanda, O.A. Odekunbi and E. Odegbaju Vol.8, No.10
18/8 stainless steel showed a cyclical trend of increase/decrease in potential from the first day
from a potential of -200mV(SCE) until the 23rd day when it settled in the potential characteristic
of bare steel(-550mV(SCE)) till the last day of test. The pH of the corrosive environment was
seen to move from acidic to neutral and to alkalinity within 5 days. This significant trend
affected the potential as well as the corrosion activity.
3.1.2 Effect of exposure time on corrosion rate
The extent of susceptibility to corrosion in natural fluids depends on the aggressiveness of
chemical reactivities, transport properties of environment, concentration of corrosion species in
the medium (pH), the metallurgy of the alloy sample and temperature of the corrosion medium9.
In the cassava fluid, the corrosion rates of all the samples were observed to be influenced
extremely by the changing pH of the corrosion environment. Corrosion rate of the 18/8 stainless
steel decreased steadily after the fifth day because the pH had changed from acidity to alkalinity
within five days. This decreasing trend continued till the last day of the test.
For the nickel plated sample, there was active corrosion activity up to a pH of 12 which was the
15th day of exposure in the corrosion environment. Passivation was observed at a pH of 12 which
was the 15th day and moved out of passivation on the 20th day with a pH of 12.5 .Thus, the
corrosion rate for the nickel plated sample went into passivity with no corrosion occurring on the
15th to 20th days when the pH hit 12 and at a pH of 12.5 renewed corrosion activity began. This is
showing that E-pH diagram for Ni-CN- prevalence diagram has a thin passive segment between
pH of 12 and 12.5 after which corrosion species become prevalent.
The unplated steel was experiencing increased high level corrosion activity up till the fifteenth
day rising from a pH of 4.0 to 12 after which there was a decrease in corrosion rate on the 20th
day remaining constant till the end of the test. This decrease corresponds to the decreased
potential from -770Mv(SCE) to -550Mv(SCE) which is characteristic of bare steel and this
potential remained steady till the end of the test.
3.1.3 Suitability of nickel coatings in a Cassava environment
It is evident that uncoated steel is unsuitable for a material of construction for cassava processing
equipment and this work tested the use of 18/8 stainless steel and nickel plated coatings as a
potential protective coating. Unfortunately, nickel appears to have renewed rapid corrosion
activity at a pH above 12.5 after a thin segment of passivity at pH of 12 to 12.5. The reason for
the relatively rapid corrosion of nickel is undoubtedly due to the easy complexing of nickel as a
cyanide species and the consequently more rapid corrosion after a pH of 12.5. Unfortunately,
results in nickel being an unsuitable material for the protection of steel in cassava fluid. 18/8
Vol.8, No.10 Corrosion Behavior of 18/8 Stainless Steel 811
stainless steel is suitable for protection in cassava environment.
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