Perevoznikov, S. S.

Poznyak, S. K.

Tsybulskaya, L. S.

BSU Digital Library

 76 
Protective ability of electrodeposited and electroless Ni–P alloy coatings on 
mild steel in saline media 
S. S. Perevoznikov, S. K. Poznyak, L. S. Tsybulskaya 
Research Institute for Physical Chemical Problems, Belarusian State University, Minsk, Belarus, 
e-mail: PerevoznikovS@yandex.ru 
Nickel phosphorus (Ni–P) alloys are of great interest because of their useful properties such 
as anticorrosive protection, wear resistance, high hardness, low friction coefficient, paramagnetic 
characteristics, catalytic activity in hydrogen evaluation reaction. Although Ni–P coatings have 
been intensively studied, their protective properties are still not completely understood [1, 2]. In 
particular, no comparison of protective ability of Ni–P alloys produced by different methods can 
be found in literature. Since the commonly used methods for Ni–P coatings production are elec-
trodeposition and electroless deposition, the present work was focused on comparing the protec-
tive properties of electroless and electrodeposited Ni–P coatings. The phosphorus content in Ni–
P coatings is known to affect strongly their structure and anticorrosion properties. Therefore we 
prepared the electrodeposited Ni–P coatings with the phosphorus content 5.5, 10.0 and 21.5 at. 
%, which had microcrystalline, amorphous-crystalline and amorphous structure respectively. 
Since in the case of electroless deposition the P content in the coatings cannot be varied by 
changing the parameters of deposition process, we prepared electroless Ni–P coatings containing 
only 10 at. % of phosphorus which had amorphous-crystalline structure. The thickness of both 
kinds of the coatings was about 20 µm. 
A systematic study of the effect of phosphorus content and the origin of Ni–P coatings on 
their protective ability on a mild steel substrate in saline media (3.5 % NaCl) was performed. 
Impedance spectroscopy was used as a powerful method for diagnostics of corrosion processes 
because of its high sensitivity to surface state in aqueous solutions. Immersion corrosion test of 
the coatings in saline solution was conducted for 1000 hours (about 40 days). During this period 
impedance spectra of the samples were periodically recorded under open circuit conditions, and 
then fitted with Nova software using corresponding equivalent circuits. 
The research revealed a great difference in protective ability of the electrodeposited Ni–P 
coatings with different phosphorus content. Calculated charge transfer resistance (Rct) of the 
electrodeposited crystalline Ni–P samples strongly fluctuated in the range of 10-40 kOhm·cm2 
during the first 10 days and then was stabilized at 10–15 kOhm·cm2. Then after 680 hours of 
testing the Rct began to rise up to 40 kOhm·cm2. This behavior can be due to the penetration of 
aggressive chloride ions though pores and microcracks of the coating to steal substrate. Unlike 
crystalline samples, the Rct of amorphous-crystalline and amorphous coatings increased rapidly 
in early hours of testing from 15 up to 200 kOhm·cm2, indicating the formation of a thin protec-
tive film on the coatings surface. Then the Rct growth rate decelerated, and after 1000 hours of 
testing the Rct of electrodeposited Ni–P (10 at. % P) was about 300 kOhm·cm2. In comparison 
with the electrodeposited Ni–P coatings, the electroless Ni–P coating containing 10 at. % P 
demonstrated very low values of the Rct (about 7–8 kOhm·cm2) during the whole period of test-
ing, evidencing active corrosion processes of the samples. 
Thus the results obtained show that only amorphous-crystalline and amorphous electrode-
posited Ni–P coatings can provide effective protection against corrosion in saline media. Electro-
less Ni–P coatings with amorphous-crystalline structure do not protect mild steel under these 
conditions due to rheir active dissolution. 
 
References 
1. A. Krolikowski, P.Butkiewicz. Electrochim. Acta (1993) 38: 1979. 
2. A. Krolikowski, B. Karbownicka, O. Jaklewicz. Electrochim. Acta (2006) 51: 6120. 


Protective ability of electrodeposited and electroless Ni–P alloy coatings on mild steel in saline media

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Protective ability of electrodeposited and electroless Ni–P alloy coatings on mild steel in saline media

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