In addition to high hardness, Ni-P coatings obtained by the electroless method are widely used due to their high corrosion resistance. The corrosion resistance depends on the content of phosphorus in the coating. Various phosphorus contents result from changes in pH in the process of obtaining layers. The layers obtained in different working conditions were tested for electrochemical corrosion. Tests were conducted on a potentiostat PGP 201. There were plotted and analyzed the anodic polarization Tafel curves for different phosphorus contents, pH conditions and timeexposure in the coating bath. Analyzing the anodic polarization curves it is noticed that low phosphorus sample shows an almost continuous corrosion process while samples with high phosphorus content have slow corrosion in the 3.5% NaCl testing medium

Gina Genoveva ISTRATE

Tamara RADU

The Annals of “Dunarea de Jos” University of Galati Fascicle IX Metallurgy and Materials Science

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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
No. 3 - 2015, ISSN 1453 – 083X 
 
 
 
CORROSION BEHAVIOR OF Ni-P COATED STEEL STRIPS 
 
Gina Genoveva ISTRATE, Tamara RADU 
“Dunarea de Jos” University of Galati, Faculty of Engineering 
e-mail: gina.istrate@ugal.ro 
 
ABSTRACT 
 
In addition to high hardness, Ni-P coatings obtained by the electroless method 
are widely used due to their high corrosion resistance. The corrosion resistance 
depends on the content of phosphorus in the coating. Various phosphorus contents 
result from changes in pH in the process of obtaining layers. The layers obtained in 
different working conditions were tested for electrochemical corrosion. Tests were 
conducted on a potentiostat PGP 201. There were plotted and analyzed the anodic 
polarization Tafel curves for different phosphorus contents, pH conditions and time-
exposure in the coating bath. Analyzing the anodic polarization curves it is noticed 
that low phosphorus sample shows an almost continuous corrosion process while 
samples with high phosphorus content have slow corrosion in the 3.5% NaCl testing 
medium. 
 
KEYWORDS: coating, electroless method, corrosion 
 
1. Introduction 
 
Ni-P chemical coatings are obtained by means 
of autocatalytic method using a nickel salt (sulphate 
or chloride) and a reductant (sodium hypophosphite) 
[1, 2]. Protective layers of Ni-P have many features 
such as hardness and high wear resistance, corrosion 
resistance, acceptable ductility [3, 4]. One of the most 
common reasons for using chemical coatings of Ni-P 
in functional applications is their excellent resistance 
to corrosion. 
As shown in Figure 1, 30% of these types of 
coating applications refer to their increased resistance 
to corrosion [5]. 
 
30%
25%
18%
11%
6%
5%
5%
Corrosion preventing  
Wear / Hardness 
Magnetic properties
Adhesion and uniformity
Conductivity
Lubrication
Other
 
 
Fig. 1. Ni-P layer applications [5] 
 
Compared to electrochemical coatings [6], 
deposits obtained by the chemical method are usually 
less porous and more resistant to corrosion (to equal 
thickness). 
In warm highly alkaline solutions, low 
phosphorus coatings are more resistant to corrosion 
than those with high content of phosphorus [7]. 
However, alloys having high content of phosphorous 
provides increased corrosion resistance in most 
corrosive environments [8]. 
The high corrosion resistance of Ni-P coatings 
is due to the formation of a nickel coating which acts 
as a barrier to oxygen diffusion onto the metal 
surface. In general, the corrosion resistance of the Ni-
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
No. 3 - 2015, ISSN 1453 – 083X 
 
P coatings is improved by increasing the content of P 
so that the layers with high phosphorus content and 
amorphous structures feature a very high resistance to 
corrosion [9]. 
 
2. Experimental research 
 
Corrosion tests were conducted using PGP 201 
potentiostat connected to an electrochemical three-
electrode cell: working electrode – sample being 
tested on the surface of which measures were made, 
counter - electrode of platinum and the saturated 
calomel electrode, Hg/Hg2Cl2, as reference electrode 
(SCE = + 241 mV/ENH). The system was connected 
to a computer with VOLTAMASTER 4 software for 
data analysis. The samples to be tested (working 
electrode) were isolated on one side prior to 
immersion into the test solution, i.e. 3.5% NaCl 
solution. 
The corrosion test of each sample begin by 
monitoring the corrosion potential (open circuit 
potential – OCP) until this has reached the stationary 
value. 
For potentiodynamic polarization tests, the 
scanning speed of 2 mV/s was used. 
Plotting and interpretation of the Tafel curves 
give information on the corrosion current density, 
polarization resistance and the corrosion rate of the 
systems tested. 
Figure 2 illustrates the polarization curves for 
three coating layers of different phosphorous 
contents. The Tafel parameters listed in Table 1 show 
close corrosion behavior for the samples code P1 and 
P3, P2 having the best resistance to corrosion. 
 
-520 -500 -480 -460 -440 -420 -400 -380 -360 -340 -320 -300
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
lo
g 
I, 
[

A
/c
m
2
]
Potential, [mV]
 P1_5.5 % P
 P2_9.5 % P
 P3_12 % P
 
 
Fig. 2. Comparative polarization potentiodynamic curves for 
Ni-P coatings with different phosphorous contents (in 3.5% NaCl solution) 
 
Table 1. Results from the potentiodynamic polarisation curves for Ni-P coatings with different 
phosphorous contents 
 
Sample 
Potential 
[mV] 
Rp 
[kohm cm2] 
I cor 
[μA/cm2] 
βa 
[mV/decade] 
βc 
[mV/decade] 
V cor 
[μm/an] 
P1 _ 5.5 % P -393.4 10.15 2.40 127 -144.5 24.73 
P2 _ 9.5 % P -422.5 16.07 1.35 124.2 -121.5 13.96 
P3 _ 12 % P -413.3 8.96 1.30 127.6 -140.0 23.66 
 
For Ni-P coatings, the lowest corrosion rate in 
NaCl 3.5% (13.96 μm/year) was obtained with the Ni 
coating having 9.5% P content and the highest value 
of the corrosion rate (24.73 μm/year for the Ni 
coating with low P content (5.5% P). 
Since the duration of the coating process affects 
the layer thickness and this further influences the 
corrosion resistance, the corrosion behavior was 
tested on the samples obtained at different times of 
immersion for obtaining coatings. 
Results are shown in Table 2 and Figure 3. 
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
No. 3 - 2015, ISSN 1453 – 083X 
 
The lowest corrosion rate in NaCl 3.5% (14.35 
μm/year) was obtained for the Ni coating achieved in 
20 minutes and the highest corrosion rate (83.73 
μm/year) for the Ni coating obtained in 10 minutes. 
It can be seen that with increasing the time of 
exposure in the chemical deposition bath, corrosion 
rate decreases with increasing layer thickness. 
 
Table 2. Results from the potentiodynamic polarisation curves for Ni-P coatings obtained at different 
times of immersion 
 
Sample Potential 
[mV] 
Rp 
[kohm cm2] 
i cor 
[μA/cm2] 
βa 
[mV/decade] 
βa 
[mV/decade] 
V cor 
[μm/an] 
P4_10 minutes -521.7 2.28 8.1459 68.7 267.8 83.72 
P5_15 minutes -454.4 2.76 6.9861 77.8 199.9 71.80 
P6_20 minutes -392.1 12.22 1.3964 104.8 105.1 14.35 
 
-600 -550 -500 -450 -400 -350 -300
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
 
 
 l
o
g
 I
, 
[

A
/c
m
2
]
Potential, [mV]
 P04_10 minutes
 P04_15 minutes
 P04_20 minutes
 
 
Fig. 3. Comparative polarization potentiodynamic curves for 
Ni-P coatings obtained at different times of immersion (in 3.5% NaCl solution) 
 
Since at different pH of the nickel plating bath 
different chemical compositions of Ni-P coatings are 
obtained, the corrosion behavior was analyzed on 
samples obtained with different values of pH, 
maintaining the other parameters constant, 
respectively duration (10 minutes) and temperature 
(83 oC). 
The best corrosion behavior as resulting from 
the polarization curves (Figure 4) is to be found with 
the samples obtained at pH = 5. At this pH value, 
high phosphorous content is achieved that enriches 
the nickel, thus increasing the coating corrosion 
resistance. Also at pH = 5 under the same conditions 
of temperature and time, thicker layers are obtained 
as compared to those at pH = 4 and pH = 3.5, 
respectively. 
Table 3 illustrates the Tafel parameters achieved 
for these samples. 
 
Table 3. Results from the potentiodynamic polarisation curves for Ni-P coatings 
obtained at different pH values 
 
Sample Potential 
[mV] 
Rp 
[kohm cm2] 
i cor 
[μA/cm2] 
βa 
[mV/decade] 
βc 
[mV/decade] 
V cor 
[μm/an] 
P7_ pH ≈5 -357.7 10.86 1.26 86.4 90.7 12.96 
P8_ pH ≈4 -347.4 9.09 1.69 99.2 98.9 17.40 
P9_ pH ≈3.5 -367.4 16.73 1.39 169.1 119.6 14.37 
 
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
No. 3 - 2015, ISSN 1453 – 083X 
 
-460 -440 -420 -400 -380 -360 -340 -320 -300 -280
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Potential, [mV]
lo
g
 I 
[

A
/c
m
2
]
 P7_pH=5
 P8_pH=4
 P9_pH=3.5
 
 
Fig. 4. Comparative polarization potentiodynamic curves for 
Ni-P coatings obtained at different pH values (in 3.5% NaCl solution) 
 
In order to explain the corrosion behavior of the 
samples analyzed, it is of interest to study and 
account for their anodic polarization curves. 
Figure 5 shows the anodic polarization curve of 
the Ni-P layer obtained at different pH's which gives 
them different contents of P, the concentration of this 
element being greater as the pH is lower. We can say 
that P5 sample has the highest phosphorus content 
and P10 sample the lowest. By analyzing the curves 
in Figure 5 it is observed that the sample with a low 
content of phosphorus (pH = 6) features an almost 
continuous process of corrosion and high phosphorus-
containing samples obtained at lower pH have a slow 
corrosion, the evolution of oxygen depolarization 
being virtually constant at P7 sample. 
 
-500 -400 -300 -200 -100 0 100 200 300 400 500 600
-10
-8
-6
-4
-2
0
lo
g 
I [

A
/c
m
2
]
Potential, [mV]
 P5_pH=3.5
 P7_pH=5
 P10_pH=6
 
 
Fig. 5. The anodic polarization curve of the Ni-P layer obtained 
at different pH values (in 3.5% NaCl solution) 
 
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
No. 3 - 2015, ISSN 1453 – 083X 
 
3. Conclusions 
 
The Ni-P layers can be distinguished in terms of 
their resistance to corrosion based on the content of 
phosphorus as given by the pH of the nickel plating 
bath and the coating thickness which varies with the 
duration of immersion, the temperature and pH of the 
nickel-plating bath. 
The best corrosion behavior is obtained for the 
Ni-P layers with 9.5% phosphorus obtained at a pH of 
5 to a minimum immersion time of 10 minutes at 85 
oC. The highest value of the corrosion rate (24.73 
μm/year) resulted by coating with Ni and low P 
content (5.5%). 
The anodic polarization curves indicate for the 
sample obtained at pH 5 a very slow evolution of the 
corrosion process as compared to the upward 
continuous evolution of the sample obtained at pH= 6 
with a low content of phosphorus. 
 
References 
 
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Nozad, Effect of lactic acid on nucleation morphology and surface 
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Corrosion Behavior of Ni-P Coated Steel Strips

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Corrosion Behavior of Ni-P Coated Steel Strips

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