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The influence of chromate quenching and chloride contamination level on the performance of the painted hot-dipped galvanized steel (duplex system).

Abstract The influences of the chromate quenching step and the surface chloride contamination levels of galvanized steel on the performance of duplex systems were studied. Steel panels were galvanized in a commercial steel bath adopting three different post-dipping procedures. A comparative study of the galvanized steel, both painted and nonpainted, was performed by electrochemical techniques. It was verified that the chloride contamination level of the galvanized steel surfaces is the main cause of duplex system failures. An explanation for the influence of the chromate quenching on the performance of duplex systems was presented.

Keywords Hot-dip galvanizing, Galvanized steel, Chloride contamination, Zinc coating, Duplex system

Introduction

Painting hot-dip galvanized (HDG) steel parts, known as a duplex system, is largely used when high corrosion performance is required in very aggressive atmospheres. The duplex system presents the synergistic effect which states that the corrosion protection provided by the combination of zinc plus paints is superior to the sum of protection afforded by zinc and by paints separately. (1-5) Despite the well-known excellent corrosion performance of the duplex system, (1-5) failures are frequently encountered due to blistering followed by detachment of paint applied on batch HDG steel structures. (3) (5-10) This kind of problem has been known for a long time and there is a consensus regarding the difficulty in obtaining good paint adhesion on batch HDG steel. There is also a consensus that water quenching or chromate quenching stages impair the paint adhesion, therefore, the literature states that these stages must be avoided. (4-6) Actually, this recommendation appears in the ASTM D 6386 standard (11) and in an American Galvanizers Association specification. (12) In Brazil, a market survey performed by the authors showed that if the galvanizers know that a steel piece will be painted after galvanizing, they do not submit that piece to a water or chromate quenching. According to the American Galvanizers Association, water quenching or chromate quenching stages may contaminate the zinc surface. (4) (5) Not much information was found regarding the nature and the source of the contaminants. On the other hand, some researchers claim that a thin layer of basic zinc chloride derived from the zinc ammonium chloride fluxing residues is the cause of the low paint performance, but this fact is supposed to occur only when wet fluxing is adopted in galvanizing processes. (6) (7) A relationship between the chloride contamination and the water or chromate quenching and, therefore, the reasons for the recommendation in avoiding water or chromate quenching in duplex system were not found in the literature.

In addition, the chromate role on the paint performance on zinc is controversial. While the hot-dip galvanizing industry recommends avoiding chromate quenching, some authors state that the chromate conversion coating on zinc or chromate ions in the paint is beneficial for paint performance. (10) (13-18)

Fragata et al. (19) studied many paint systems applied on HDG steels aiming at identifying the ones which presented the best corrosion performance among them. However, all the studied systems presented premature blistering and paint adhesion failures. According to the authors, some of the studied paints were known to be suitable for HDG steel and all of them were applied very carefully adopting good surface cleaning laboratory practices. In order to identify the causes of the premature blistering and failures, the authors conducted some tests and analyses and detected chloride levels within the range from 3 up to 7 [micro]g [cm.sup.2] on the surface of HDG steel panels. (8) Based on these results, they concluded that the presence of soluble salts, most likely chlorides, coming from the flux used in the hot-dip galvanizing process, caused the premature failures.

Morcillo et al. (20) studied the performance of paints applied on new batch HDG steel panels and on batch HDG steel panels aged in urban and rural atmospheres and in humidity, in salt spray and in sulfur dioxide chambers. They concluded that the presence of soluble salts (chlorides and sulfates) at the paint/HDG steel interfaces caused blistering and delamination.

Fuente et al. (21) also conducted a study in order to verify the influence of soluble salts on zinc surfaces. They contaminated zinc sheets by spraying them with solutions containing different amounts of chlorides and sulfates dissolved in water and methanol. After drying the zinc sheets, they were painted and submitted to accelerated and nonaccelerated corrosion tests. Non-contaminated zinc sheets were used as a reference. They verified that the presence of soluble salts stimulated the osmotic blistering and later the corrosion of zinc.

The above studies mention soluble chloride compounds. However, in a previous work (22) conducted by the authors, a nonsoluble chloride compound, the simonkolleite, was found on the surface of batch HDG steel samples obtained in a commercial bath. In this study, a field investigation was conducted in order to identify the source of the mentioned chloride contamination. It is known that, in the batch hot-dip galvanizing process, which is a multistage process, there are two possible sources of a chloride contamination: the hydrochloric acid pickling stage and the zinc ammonium chloride fluxing stage. In addition, in some hot-dip galvanizing plants, solid ammonium chloride is thrown (ammonium chloride powdering) over galvanized pieces during their removal from molten zinc baths in order to remove the zinc excess. In Brazil, this practice is quite common. The latter is another possible source of chloride contamination.

The above mentioned study showed that, among the three possible sources of chloride contamination, the ammonium chloride powdering stage is the main one. It was also verified that the contamination occurs indirectly. The ammonium chloride suffers decomposition at temperatures equal or higher than 372[degrees]C forming ammonia (N[H.sub.3]) and hydrogen chloride according to the reaction:

N[H.sub.4]Cl(s) - N[H.sub.3](g) + HCl(g)

Once the HDG surfaces present a temperature between 400 and 450[degrees]C when they are removed from the molten zinc bath, the decomposition of the ammonium chloride is likely to occur during powdering with ammonium chloride which can be clearly observed in the industrial facilities because white fumes form during this step, as shown in Fig. 1. This causes a significant contamination of the atmosphere of industrial hot-dip galvanizing facilities. The suspended NH3 and hydrogen chloride can recombine in the atmosphere and form small particles of ammonium chloride which can settle on and contaminate the chromate bath or the water used for quenching stages as well as all galvanized pieces lying on the floor. Analyses performed with chromate bath showed a negligible amount of chloride in new baths and high concentration of chloride in old baths. In addition, a high chloride contamination of HDG panels stored for a long period in the galvanizing facilities was found. The main conclusion of the study was that the contamination most likely occurs as a consequence of the deposition of the recombined ammonium chloride which may occur directly on the finished products, stored in the galvanizing facilities, or directly in the baths used for quenching. The direct contact between ammonium chloride and the hot zinc surfaces is not the source of contamination because this salt decomposes when in contact with the hot panels.

[FIGURE 1 OMITTED]

This study attempted to verify whether the recommendation to avoid the chromate quenching by the literature is due to the chromate itself or is due to the influence of the chloride contamination that occurs at the chromate quenching stage.

Experimental Methods

Carbon steel panels, 100 x 150 x 4 mm, were HDG in a molten commercial zinc bath adopting three different after dip treatments (three lots) attempting to obtain different chloride contamination levels and chromium levels. For each condition, 200 panels were obtained. The three conditions were as follows:

Lot NP: air cooled (without quenching);

Lot CrO: chromate quenched in a very old bath;

Lot CrN: chromate quenched in a new bath.

The chloride content of the chromate baths was analyzed through a titrimetric method using standardized silver nitrate. The old bath presented 365 mg [L.sup.-1] and the new one 1 mg [L.sup.-1] of chloride.

The surface of each lot was fully characterized aiming at determining the surface content of chloride and chromium. Chloride was determined, in triplicate, through a boiling method: aqueous extracts were by boiling a HDG panel (100 x 150 mm) in 4 L of distilled water for 60 min. After the extraction, the volume of the extract was reduced to 250 mL and its chloride content was determined by an analytical analysis. The obtained value was divided by the panel area. The detailed description of the boiling method was presented elsewhere. (22)

The superficial chromium content was obtained through a semi-quantitative energy dispersive microanalysis (EDS) using a scanning electron microscope (SEM). The obtained results were as follows:

Lot NP: chromium was not detected; 2.1 [micro]g [cm.sup.-2] of chloride were detected;

Lot CrO: 0.23% of chromium and 13.4 [micro]g [cm.sup.-2] of chloride were detected;

Lot CrN: 0.11% of chromium and 5.6 [micro]g [cm.sup.-2] of chloride were detected.

The composition of the compounds found on the HDG surfaces were determined by a X-ray diffraction (XRD) technique, using a Rigaku RINT 2000 diffractometer. Simonkolleite--Z[n.sub.5][(OH).sub.8] [Cl.sub.2] [H.sub.2]O, basic zinc chloride--was detected on the surface of the post-quenched panels (Lot CrO and Lot CrN).

For an electrochemical characterization of HDG surfaces, impedance measurements were obtained. For all measurements, a PAR 273A potentiostat with Softcorr II and a frequency PAR analyzer were used. A saturated calomel electrode (SCE) was used as a reference and a 54 c[m.sup.2] platinum basket was used as an auxiliary electrode. All the measurements were carried out in a 0.5 mol [L.sup.-1] naturally aerated [(N[H.sub.4]).sub.2] S[O.sub.4] solution which was placed in a cylindrical glass tube attached to the surface of the HDG panels, using a silicon-base binder. The exposed HDG surface area was 16 [cm.sup.2]. Three measurements were obtained for each panel.

The impedance measurements were done after the open circuit potential stabilization which took place after some minutes. The measurements were performed under polentiostatic control at the open circuit potential. The scanned frequency range was from [10.sup.4] to [10.sup.-2] Hz and the amplitude was 10 mV from the open circuit potential. Ten points were acquired for each frequency decade.

Several panels of each lot were cleaned lightly using an organic solvent. This cleaning method was not able to remove either soluble or insoluble inorganic compounds from the surface of the panels and did not cause any kind of contamination of the HDG surfaces. These conclusions were based on semi-quantitative EDS performed using a SEM.

After the cleaning, the panels were painted by brushing with one coat of a two-component isocyanate epoxy primer (dry coating thickness of about 20 [micro]m). After the curing, a topcoat of a two-component polyamide epoxy paint (dry coating thickness of about 40 [micro]m) was applied. Both paints were analyzed. These analyses showed that they were in agreement with their specifications.

Six painted panels of each lot were exposed to a humidity chamber according to ASTM D 2247 for 1344 h (56 days). The adhesion of the paint layer was verified before and after the exposition to the humidity chamber according to ASTM D 3359.

Small samples with an area of 40 x 40 mm were taken from a panel of each lot and submitted to the same above mentioned cleaning and painting procedures. These small painted samples were immersed in distilled water. Periodically, they were removed from the distilled water and were placed in a chamber maintained at 98% RH. Then, corrosion potential maps of a previously selected 1 cm2 area of the zinc surface under the paint were obtained for each lot using a Kelvin Probe. Before each measurement, the probe was calibrated using a saturated Cu/CuS[O.sub.4] reference electrode.

Results and discussion

Nonpainted HDG panels

Figure 2 shows the impedance diagrams for the HDG steel samples in a 0.5 mol [L.sup.-1] (N[H.sub.4])2S[O.sub.4] solution. From the Bode phase diagram (Fig. 2a), two time constants are observed, one in the range of high frequencies and another in the range of low frequencies. As cited in the literature, (23) the former is related to the corrosion product layer for the nonchromate quenched sample (Lot NP) and also related to the corrosion product doped with hexavalent chromium for the chromate quenched samples (Lot CrO and Lot CrN). The latter time constant is related to the corrosion process.

[FIGURE 2 OMITTED]

The resistive component values at 10 mHz, obtained from the Nyquist diagram (Fig. 2c), were as follows: 23 [ohm] for Lot NP, 288 [ohm] for Lot CrO, and 115 [ohm] for Lot CrN. The resistive component value at low frequencies is approximately the value of polarization resistance around the open circuit potential which in turn is inversely proportional to the corrosion rate. Thus, these results indicate that the nonchromated sample is the most active. A similar conclusion was obtained by Magalhaes et al. (23) These authors compared the corrosion performance of a nonchromated electrogalvanized zinc coating with chromated ones through traditional immersion tests and through electrochemical impedance measurements. They verified that, at 3 mHz, the resistive component value of a nonchromated zinc coating is lower than the values obtained for chromated zinc coatings and that the higher the chromium content on the zinc surface, the higher the resistances. By comparing the results obtained from the traditional immersion tests with those obtained from the electrochemical impedance measurements, they concluded that the relative corrosion resistance of chromated and nonchromated electrogalvanized zinc coatings can be verified using the resistive component values at a fixed low frequency. They also concluded that, for higher frequencies, a correlation does not exist between the resistive component of the impedance and the features of the chromate treatment.

In this study, the sample with the intermediate chromium content (Lot CrN) presented a lower resistive component (115 [ohm]) at a fixed low frequency than the sample with the highest chromium content (Lot CrO--287 [ohm]) which is in agreement with Magalhaeset al. (23)

Painted HDG panels

Table 1 shows the results of the X-cut tape adhesion test, performed according to ASTM D 3359, before and after 1344 h of exposure to a humidity chamber. The superficial aspect of each tested panel at the X-cut region is also shown. In this test, the adhesion evaluation was done measuring the detachment along the incision (X detachment, i degree) and the detachment on intersection (Y detachment, / degree). The higher the i and j values, the lower the paint adhesion.

From Table 1, it is verified that the paint adhesion had been very good before the exposure to a humidity chamber. However, a complete loss of adhesion is observed after the humidity test. In addition, blister formation was observed during the exposure. After the test, the paint layer of the blistered areas was carefully removed and the zinc surface was inspected using a magnifying lens. White corrosion products were observed beneath all of the inspected blisters.

Figure 3 shows the corrosion potential maps, obtained by using a Kelvin Probe, for the painted Lot-NP HDG sample. The measurements were performed over the same 1 [cm.sup.2] area of the sample surface and after 4, 13, 22, and 29 days of immersion in distilled water. From this figure, it can be observed that, after 4 days, a great portion of the analyzed area presents corrosion potential values between the range from -300 to -400 m[V.sub.HE] (HE, hydrogen electrode) and some small areas with lower values (up to -500 MVHE). The large range of values for the corrosion potential can be attributed to the roughness of the zinc surface. The samples for a Kelvin Probe mapping should be as flat as possible because the measured values depend on the distance between the probe tip and the sample surface. As the HDG samples of this study were steel panels coated with zinc in an industrial facility and the paint was applied by brushing, their surface does not present the desired flatness.
Table 1: X-cut tape adhesion tests of the painted panels before
and after 1344 h (56 days) of exposition to the humidity chamber

     X-cut tape
     adhesion

Lot  Before exposition      After exposition to     Note
     to humidity            humidity chamber
     chamber

NP   [X.sub.0][Y.sub.0]     [X.sub.4][Y.sub.4]      Blisters
                                                    are formed
                                                    after few
                                                    days of
                                                    exposition

CrO  [X.sub.0][Y.sub.0]     [X.sub.4][Y.sub.4]      Blisters
                                                    are formed
                                                    after few
                                                    days of
                                                    exposition

CrN  [X.sub.0][Y.sub.0]     [X.sub.4][Y.sub.4]      Blisters
                                                    are formed
                                                    after few
                                                    days of
                                                    exposition


[FIGURE 3 OMITTED]

Using a Kelvin Probe, a corrosion potential value of -300 m[V.sub.HE] was obtained by Williams and McMurray (17) at an HDG/humid atmosphere interface. The same value was obtained over an intact paint surface applied on HDG steel samples exposed to the same humid atmosphere. These authors slated that this value corresponds to a lack of bulk electrolyte at the zinc/ paint interface, the zinc surface being covered with a passive hydroxide/oxide layer. By comparing the results obtained by Williams and McMurray (17)with Lot NP, it is possible to conclude that the range from -300 to -400 m[V.sub.HE] should represent the corrosion potential of nonactive (passivated) zinc surfaces.

The lower corrosion potential values at some points of the surface of the painted Lot-NP sample after 4 days of immersion in distilled water may be attributed to a localized electrochemical activity at the zinc/ paint interface which determined the corrosion of the zinc surface.

From Fig. 3, it can be observed that, after 13 days, the area of the active points increased. At this time, small blisters were formed on the painted surface. Most probably, the blisters were formed due to the formation of zinc corrosion products.

After 22 days, cathodic and anodic areas were clearly verified. The areas with lower corrosion potential values (-700 m[V.sub.HE]) correspond to blistered areas and the areas with higher corrosion potential values (-200 m[V.sub.HE]) correspond to the cathodic areas. It is important to emphasize that the corrosion potential of the latter areas increases with immersion time (see Fig. 3--22 and 29 days). This fact can be attributed to the alkalinization of the cathodic areas which, in turn, increases the efficiency of the hydroxide/oxide passive film formed on the cathodic areas of the zinc surface. Delamination studies of painted HDG surfaces conducted by Furbeth and Stratmann (24) have estimated the underfilm electrolyte pH to assume values between 10 and 11. In this range of pH, the zinc surface, in contact with pure water, becomes passive. (25)

After the test, the paint layer of the blistered areas was carefully removed and the zinc surface was inspected using a magnifying lens and an SEM. White corrosion products were found at these regions as can be observed in Fig. 4. This result confirmed the assumption that the blisters were formed due to the formation of zinc corrosion products. The EDS analysis did not detect chloride ions al the surface of zinc either on the blistered area or on the nonblistered area (Fig. 4) which confirmed the lowest surface contamination of this sample with chloride (2.1 [micro]g [cm.sup.-2]).

[FIGURE 4 OMITTED]

Figure 5 shows the corrosion potential maps of the painted Lot-CrN sample. The measurements were performed over the same 1 [cm.sup.2] area of the sample surface and after 4, 18, 24, 27, and 31 days of immersion in distilled water. From this figure, it can be observed that after 4 days, a great portion of the analyzed area presented corrosion potential values between the range from 0 to -300 m[V.sub.HE] and one small area with a very low value (-550 m[V.sub.HE]). As Lot NP, the large range of potential values may be attributed lo the roughness of the zinc surface.

Comparing the corrosion potential values of Lot CrN (from 0 to -300 m[V.sub.HE]) with the range of Lot NP (from -300 to -400 m[V.sub.HE]), it is possible to conclude that the surface of Lot CrN was less active. The higher values of the corrosion potential range of Lot CrN may be attributed to the presence of chromates. Williams and McMurray, (17) studying the corrosion behavior of zinc in a chloride solution, verified that the addition of Cr[O.sub.4.sup.2-] in the electrolyte caused an increase of the corrosion potential and a significant decrease of the corrosion of zinc.

As with Lot NP, it is possible to suppose that the lower corrosion potential values, observed in Fig. 5 after 4 days of immersion, represent The active sites (higher electrochemical activity). Once again, the active area increased with immersion time (see Fig. 5) but, in the case of Lot CrN, the blisters appeared after 27 days which again indicates that Lot CrN is less active.

[FIGURE 5 OMITTED]

These results indicate that the presence of a thin chromate layer on the HDG surface of Lot CrN (0.11% of Cr) compensated its surface chloride contamination (5.6 [micro]g [cm.sup.-2]) and retarded the formation of zinc corrosion products which, as stated before, are the cause of the formation of the blisters. This hypothesis was confirmed once again through visual inspection of the zinc surface after the removal of the paint layer of the blistered area. White corrosion products were found at these regions. In this case, the local EDS analyses detected chloride ions on the blistered areas. No chloride ions were found on the zinc surface not covered with corrosion white products (nonblistered area), as shown in Fig. 6.

[FIGURE 6 OMITTED]

Figure 7 shows the corrosion potential maps of a painted Lot-CrO sample, after 4, 18, and 24 days of immersion in distilled water. From this figure, it can be observed that, after 4 days, the scanned area presented corrosion potential values in the range from -400 and -600 m[V.sub.HE]. These values are lower than the values of Lot NP obtained after 4 days. In this case, the presence of chromate layer on the HDG surface of Lot CrN (0.26% of Cr) did not compensate its high level of surface chloride contamination (13.46 [micro]g [cm.sup.-2]). Most likely, the latter dissolved, or even prevented the formation, of the passive layer on HDG surfaces, making them more active. Despite the mentioned lower corrosion potential values of Lot CrO, its performance was slightly better than Lot NP as the blisters appeared only after 18 days of immersion, i.e., 5 days later than Lot NP. Probably, there should be a competition between the beneficial effects of a chromate coating with the detrimental effects of the chloride contamination. It is worth mentioning that the blistered area of this sample also presented white corrosion products and a chloride contamination level higher than Lot CrN, as shown in Fig. 8.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

These results indicate that the widespread belief that the detrimental influence of a chromate quenching is not a consequence of the presence of a chromate conversion film by itself but is determined by the contamination of the HDG steel surfaces with chloride ions.

As discussed in the "Introduction" section, the atmosphere of a hot-dip galvanizing facility may be highly contaminated with ammonium chloride particles which settle down and contaminate the chromate quenching bath which in turn becomes a potential source of a chloride contamination. Therefore, as the chromate bath becomes older, its chloride content increases and, consequently, the surface contamination of HDG steel surfaces with chloride increases. The higher contamination with chloride impairs the beneficial effect of the chromate quenching.

Conclusion

The chloride contamination of the batch HDG surfaces is the main cause of the poor performance of duplex systems. The pointed deleterious effect of the chromate quenching on the performance of the duplex systems is not a consequence of the presence of a chromate layer by itself. Rather, it is a consequence of the contamination of chromate bath with chloride ions coming from the galvanizing facility atmospheres. A chromate quenching in a chloride-contaminated bath produces chloride-contaminated surfaces which in turn impair the performance of the duplex system.

Acknowledgments The authors thank Dr. Daniel de La Fuente from CENIM, Madrid, Spain and Professor Dr. Michael Rohwerder from Max-Planck Institute fur Eisenfurschung GmbH, Dusseldorf, Germany for the laboratorial and the technical support for the corrosion mapping using a Kelvin Probe.

References

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G. M. Bragagnolo, Z. Panossian, N. L. de Almeida, M. B. de Almeida

Corrosion and Protection Laboratory of Institute for Technological Research (IPT), Av. Prof. Almeida Prado, No. 532, 05508-901 Sao Paulo, SP, Brazil

e-mail: gbraga@ipt.br

Z. Panossian

e-mail: zep@ipt.br

N. L. de Almeida

e-mail: neusval@ipt.br

M. B. de Almeida

e-mail: marcioa@ipt.br

Z. Panossian, J. V. Ferrari

Polytechnic School of Sao Paulo University (EPUSP), Av. Prof. Luciano Gualberto, No. 380, 05508-900 Sao Paulo, SP, Brazil

J. V. Ferrari

e-mail: jeanferrari@uol.com.br

M. C. Andreoli

Paulista Company of Electric Energy Transmission (CTEEP), Rua Casa do Ator, No. 1155, 04546-004 Sao Paulo, SP, Brazil

e-mail: mandreoli@cteep.com.br

F. de L. Fragata

Research Center of Electrical Energy (CEPEL), Av. Horacio Macedo, No. 354, 21941-902 Rio de Janeiro, RJ, Brazil

e-mail: fragata@cepel.br

DOI 10.1007/s11998-011 -9349-7
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Date:Oct 1, 2011
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