Investigation of the inhibiting effect of environmentally friendly cerium-containing conversion films on the corrosion of zinc coatings

In the present work, the inhibiting effect of environmentally friendly cerium and phosphorus-based conversion films on zinc corrosion in 5% NaCl and 1 N Na2SO4 test media was investigated. The films were prepared by immersion and stirring in the passivating/converting solution for time periods of 3,5,7, 10 min., respectively. The chemical composition of the films was determined by EDX analysis. The results of the electrochemical studies showed that in 5% NaCl medium, cerium- and phosphorus-based films exhibited high inhibition effect only during the first days of polarization resistance (Rp) measurement. In 1 N Na2SO4 medium, the Rp of the conversion films was up to 2.5 times higher than that of pure zinc coatings throughout the test period.


Introduction
One of the most common methods for protecting iron from corrosion is the galvanizing.To improve the protective properties of the zinc layer, surface treatments such as phosphating, chromating, chromiting are used in practice [1,2].In connection with European Union directives prohibiting the application of Cr 6+ , the investigations in the recent years have been directed towards the development of zinc passivating compositions based on molybdenum, vanadium, cerium, etc. [1] The property of cerium to form oxide, hydroxide or mixed layers on zinc over a wide pH range makes it a suitable substitute for toxic Cr 6+ [3,4].Cerium-based passivating formulations include an oxidizing agent (H2O2) and acids (nitric, malic, etc.) [5,6,8].In order to increase the corrosion resistance of the resulting cerium films, the addition of various compounds to the basic passivation compositions including Na2SO4, NaNO3, citric acid and some organic additives such as 2-butene 1,4 diol propoxylate has been investigated [6][7][8][9].
The idea of the present study was to develop an environmentally friendly composition for the preparation of cerium conversion films with phosphorus, the latter component also known to inhibit the corrosion of the zinc.The aim was to obtain conversion (passive) films on zinc and to characterize their corrosion behavior in model test media of 5% NaCl and 1 N Na2SO4, simulating local and industrial atmosphere corrosion, respectively.
The zinc coatings were deposited on 1x2 cm low carbon steel substrates (total area 4 cm 2 ).In order to improve adhesion prior to the electrochemical deposition process, a preliminary preparation of the metal surface was carried out including chemical degreasing in hot alkaline solution and etching in 36 wt.% HCl diluted with distilled water in a 1:1 ratio.After preparation, the specimens were rinsed with distilled water and dried.The thickness of the deposited coatings was about 10-12 µm.
The surface morphology and chemical composition of the zinc/conversion film systems were determined using a scanning electron microscope (SEM) with an EDS analysis device manufactured by JEOL, model JSM 6390.The thickness of cerium conversion films was determined by SEM analysis on cross-sections of zinc specimens treated for 3 and 10 minutes in the passivation composition.
Electrochemical corrosion studies were carried out in model corrosion media of 5% NaCl and 1 N Na2SO4 at pH ~ 6.3 and at room temperature.Two methods were applied: -Potentiodynamic polarization curves, at a potential scan rate of 1 mV/s -VersaStat 4 (PAR) apparatus was used.From the polarization curves, the main parameters of electrochemical corrosion like corrosion potential (Ecorr) and corrosion current density (icorr) were determined.
-Measurement of polarization resistance (Rp) for a period of 30 days -the test was carried out on "Corrovit" apparatus at polarization applied in the range of ±25 mV with respect to the corrosion potential (Ecorr).

Surface morphology and chemical composition
Analysis of the surface morphology of the specimens passivated for 3 min (figure 1 a) showed that no visible cracks were observed.A cracked Zn+CeP layer was obtained after treatment for 10 min (figure 1 b).In table 1 the results of EDS analysis of zinc coatings after treatment in the cerium-containing passivating composition are presented.It can be seen that with increasing of the passivation time, the content of cerium, oxygen and phosphorus in the film increases.This can be associated with an increase in the thickness of the cerium-phosphorus-oxide layers.
To estimate the thickness, cross sections were prepared of zinc samples passivated for 3 and 10 min.SEM analysis showed that at these passivation times, the thickness of the films was 0.88 µm and 1.92 µm, respectively (figure 2).

Potentiodynamic polarization (PDP) curves
From the potentiodynamic polarization curves, the main parameters of electrochemical corrosioncorrosion potential (Ecorr) and corrosion current density (icorr) were determined.The results showed that in a model medium of 5% NaCl solution, the corrosion potential of passivated zinc coatings (table 2) was more positive than that of pure zinc coating by about 0.01 -0.02 V.As the cerium content of the conversion film increases (with increasing passivation time) (table 1), the corrosion current of the passivated zinc coatings decreases 2 -3 times compared to that of the pure zinc coating.
Table 2. Corrosion parameters of Zn and Zn+CeP systems in 5% NaCl solution.
In 1 N Na2SO4 solution, the corrosion potentials of passivated zinc coatings are very close in value to that of pure zinc ones (table 3).The values of the corrosion current densities of the passivated specimens were also close to those of the pure zinc coating.From the analysis of the main parameters of electrochemical corrosion, it can be seen that a greater inhibitory effect of cerium-containing conversion films is observed in a 5% NaCl medium simulating local corrosion conditions.
PDP method is an express method for evaluating the corrosion of a material in a given environment.The icorr and Ecorr are basic parameters of the corrosion process, but they do not provide information on the type, adhesion and solubility of corrosion products, which can significantly affect the corrosion behaviour of a material in a given medium, when studying stationary corrosion over an extended period of time.

3.3.Polarization resistance (Rp)
The main criterion for the protective ability of electroplated coatings is the polarization resistance (Rp), whose value (according to the Stern-Gehry equation) is inversely proportional to the value of the corrosion current [10].High values of Rp correspond to low values of corrosion current (low corrosion rate), and vice versa.In a model medium of 5% NaCl solution the Zn+CeP(5 min), Zn+CeP(7 min) and Zn+CeP(10 min) systems (figure 3 a, curves 3, 4, 5) demonstrated significantly higher polarization resistance values compared to the pure zinc coating (figure 3 a, curve 1) during the first 1-2 days of measurements.These results correspond with the icorr values in table 2. After the 5-th day, the Rp values of the passivated specimens became lower than those of the pure zinc coating.The most probable reason for this observation seems to be the presence of cerium oxide and phosphate-based corrosion products, which inhibit the formation of zinc hydroxide chloride, the latter well-known with its low solubility value and well-expressed barrier properties [2].
The results obtained from the polarization resistance method in the model medium of 1N Na2SO4, showed that throughout the test period of 30 days, the values of this parameter for the Zn+CeP(3 min) and Zn+CeP(10 min) systems (figure 3 b, curves 2 and 4) were significantly higher (up to 2.5 times) compared to Rp of the pure zinc coating (figure 3, curve 1).For the Zn+CeP (7 min) system (figure 3 b, curve 3) this is true for the first 3 days of measurements and after the 25-th day of measurements.Apparently, in the milder medium of 1 N Na2SO4 some of the conversion cerium films containing phosphorus show very good protective properties, in terms of retarding the dissolution of the zinc coating.

Coating type
Corrosion potential Ecorr (V)

Conclusions
A new eco-friendly composition for the preparation of conversion films on zinc based on CeNO3 and H3PO2 was developed.As the processing time in the passivation solution increased the thickness of the conversion films also increased up to 1.92 µm.At a treatment time of 10 min, the amounts of Ce and P in the layer were 4.02 wt.% and 1.12 wt.%, respectively.
In a 5% NaCl medium, the obtained conversion films showed a high inhibition effect (low icorr and high Rp values) against zinc corrosion only during the first 2 days of electrochemical studies.
In a model medium of 1 N Na2SO4, the Zn+CeP (3 min) and Zn+CeP (10 min) systems exhibited higher Rp values (up to 2-3 times) throughout the entire study interval.
This provides a reason to recommend them as corrosion inhibiting zinc layers in media without presence of chloride ions.

Table 3 .
Corrosion parameters of Zn and Zn+CeP systems in 1 N Na2SO4 solution.