Analysis of influence of snow melting agents and soil components on corrosion of decorative chrome plating.
The dissolution and exfoliation of chromium plating specific to Russia was studied. Investigation and analysis of organic compounds in Russian soil revealed contents of highly concentrated fulvic acid. Additionally, it was found that fulvic acid, together with CaCl2 (a deicing agent), causes chromium plating corrosion. The fulvic acid generates a compound that prevents reformation of a passivation film and deteriorates the sacrificial corrosion effectiveness of nickel.
CITATION: Kajiyama, Y., Obata, T., Sugimoto, T., Nakamura, M. et al., "Analysis of Influence of Snow Melting Agents and Soil Components on Corrosion of Decorative Chrome Plating," SAE Int. J. Mater. Manf. 9(3):2016.
As the global automobile market grows, an increasing number of cars are used in areas where there is a highly corrosive environment. Decorative chromium plating for automobile exterior coatings is used on parts of the vehicle that are important with regard to the strong impact that they have on brand image and design properties. For this reason, improving anti-corrosiveness and maintaining a high quality appearance is critical.
In Russia, where the automobile market is expanding, chromium plating sometimes suffers from a specific type of corrosion. Calcium chloride (CaCl2), a de-icing agent, is sprayed in large quantities during winter in Russia, and is a suspectedcause of this corrosion. Calcium chloride produces chloride ions at high concentration, giving chromium a lower potential than that of nickel, and probably causing the creation of a nickel-chromium cell containing a dense calcium chloride solution that dissolves the chromium .
An experiment to reproduce chromium dissolution-corrosion was performed by simulating a brine mud environment using calcium chloride and kaolin , resulting in dissolution-corrosion of the chromium being reproduced. However, the reason that the corrosion occurs specifically in Russia is not yet clear, since calcium chloride is sprayed for the purpose of melting snow in many other areas.
A study was carried out to explore the causes of the chromium corrosion that specifically occurs in Russia, and the mechanism of corrosion of the chromium plating was determined. This report describes the results.
SURVEY OF RUSSIAN SOIL
Many roads in Russia are not paved, and therefore a large amount of soil deposits on the surface of the vehicle body. Since a large quantity of de-icing agent is sprayed in winter when calcium chloride is scattered, it comes into contact with the vehicle and adheres strongly to the vehicle body following repeated cycles of drying and wetting in the air, and consequently causes the chromium plating to become corroded. A survey of de-icing agent components and types concluded that there are no large differences between those in Russia and other areas. In this study, a different survey was conducted focusing on Russian soil.
First, corrosive inorganic compounds, soluble ions, and particle size distributions in the soil were analyzed, and the properties of Russian soil were compared with those of the soil in other countries. However, no special properties were found in Russian soil.
Next, in the course of a geological and pedological survey, a report was found in a soil database  which stated that Russian soil belongs to an organic soil area, and that its organic soil components are mainly composed of bulky organic and humic organic matter, humic substances being especially reactive with metals .
Humic substances include two types of organic acids: fulvic acid (Figure 1) and humic acid (Figure 2). Figures 1 and 2 below illustrate their structure .
Fulvic and humic acid are acidic organic compounds contained in peat and lignite, which are types of humic soil. From a survey on the difference between fulvic and humic acid, it is known that fulvic acid dissolves in both acidic and neutral solvents, and that all of its functional groups give rise to cation exchange reactions. In addition, it forms a complex with metals .
Given the properties of fulvic acid, it is probable that the corrosion of chromium plating is caused by a sequence of reactions such as the following: the calcium chloride sprayed in large quantities in Russia and the fulvic acid in Russian soil give rise to an ion exchange reaction which generates hydrochloric acid and thereby reduces the pH value; fulvic acid bonds with chromium (to form a complex) giving rise to changes in chromium passivation film properties; and changes occur in the sacrificial protection effect of the plating film.
Based on the soil survey and the properties of fulvic acid, and assuming that the proposed mechanism of corrosion is responsible, the organic soil areas (Figure 3) listed in the soil database , highly ion-exchange prone soil areas (Figure 4), and highly acidic soil areas (Figure 5) were examined. The results show that Russian has many areas satisfying all these conditions, with many located in Moscow.
Soil samples were collected from some of the areas in Russia, America, Canada, and Japan shown in Figures 3, 4, and 5, and analysis was performed. Figure 6 shows the results. Up to 11 mass% of fulvic acid was detected in the soil from Russia. This shows that Russian soil contains more fulvic acid than that in other areas.
The soil survey and analysis revealed that high fulvic acid content is a property specific to Russian soil. The authors performed the following experiments in order to examine the relationship between fulvic acid and the corrosion of chromium plating.
Pre-Test: Comparison between the Effect of Fulvic Acid and Humic Acid on the Corrosion of Chromium Plating
Since fulvic and humic acid, which are humic substances, possess different properties, it is probable that they have a different effect on the corrosion of chromium plating. In order to select the more corrosive acid for the test, a pre-test was conducted to discover whether fulvic or humic acid has the larger effect on corrosion.
The test pieces were trivalent chromium plating films made using a trivalent chromium plating bath with chromium sulfate as the source.
A solution of calcium chloride and fulvic acid and a solution of calcium chloride and humic acid were prepared. Each of these test solutions was applied to each test piece, which was then dried.
The raw fulvic acid used here was artificially extracted from a cultured plant, and the humic acid reagent was made by Nacalai Tesque.
Two sets of base solution were prepared, each consisting of ten grams of calcium chloride dissolved in 50 mL of water, and then 30 g of fulvic acid was added to one and 30 g of humic acid to the other to give the test solutions.
The test pieces to which solvent had been applied were dried in a 60[degrees]C 30% RH environment for two weeks. Following this, the dried mud was removed and the chromium plating surfaces visually examined.
Main Test: Comparison of Chromium Plating Corrosion Conditions
The organic acid which was determined in the pre-test of the previous section to be the more corrosive was selected, and this acid was used to test the effect on the corrosion of chromium plating.
The test pieces were covered with a trivalent chromium plating film created using a trivalent chromium plating bath with chromium sulfate as the source.
The test pieces went subjected to three types of corrosion test conditions, as follows: (1) chromium plated automobile parts were installed on a vehicle, which was then run for monitoring purposes for two years in Moscow, Russia; (2) 10 g of calcium chloride was dissolved into 50 mL of water, 30 g of kaolin was mixed in, the resulting solution was applied to the test pieces, and the test pieces were then placed in a 60[degrees]C, 30% RH environment for two weeks; and (3) 10 g of calcium chloride was dissolved into 50 mL of water, and, after 30 g of kaolin had been added, 11 mass% of the organic acid- the same as the percentage result obtained from analysis of Russian soil -was mixed in and the resulting mixture applied to the test pieces, which were then placed in a 60[degrees]C, 30% RH environment for two weeks.
The mud was removed from these test pieces after the test had ended. Elemental analysis of the corroded areas was performed with an electron probe micro-analyzer (EPMA), test piece cross-sections were observed with a field emission scanning electron microscope (FE-SEM), and test piece cross-sections and corresponding element distributions were analyzed using a scanning electron microscope (SEM) and its ancillary energy dispersive X-ray analyzer (EDX)
Pre-Test: Comparison of the Effect of Fulvic Acid and Humic Acid on the Corrosion of Chromium Plating
Figure 7 shows the effect of fulvic acid and humic acid on corrosion of the chromium plating.
The tests using fulvic acid produced corrosion of the chromium plating, while those using humic acid produced only slight corrosion at the borderline between the applied mud and chromium film, with corrosion over the whole area not occurring. This shows that fulvic acid contributes more to the corrosion of chromium plating than humic acid. For test pieces tested in the same manner after application of a solution containing only fulvic acid or only calcium chloride, no corrosion was found. These results show that the presence of both fulvic acid and calcium chloride are required for the occurrence of this specific type of corrosion.
Main Test: Comparison of Chromium Plating Corrosion Conditions
EPMA elemental analysis of the chromium coating and SEM-EDX observation of cross-sections of the corroded areas were conducted using the following samples: (1) the trivalent chromium plated part installed in the monitoring vehicle in Russia, (2) a trivalent chromium plated test piece painted with brine mud (a solution of calcium chloride and kaolin), and (3) a trivalent chromium plated test piece painted with brine mud mixed with fulvic acid.
EPMA Elemental Analysis of Chromium Plating Surface
Figures 8, 9, and 10 show the results of EPMA surface element analysis of test pieces (1) to (3).
For all of the test pieces (1) to (3), no chromium was detected in the corroded areas, indicating that the chromium plating film had disappeared, exposing the underlying nickel layer.
FE-SEM Observation of Corroded Cross-Sections
Figure 11 shows the results of cross-sectional FE-SEM observation of the corroded areas of test pieces (1) to (3).
For test piece (1), not only is the chromium plating film corroded, but the nickel-plating film is also corroded along the interface with the chromium film. Test piece (2) exhibits corrosion of the chromium plating film only. Test piece (3) shows a similar pattern of corrosion to that of test piece (1).
These cross-sectional observation results show that, due to the presence of both fulvic acid and calcium chloride, corrosion similar to that of the chromium plated part of the monitoring vehicle in Russia was generated in the condition reproduction tests, and that promotion of corrosion occurred not only in the chromium plating film but also in the nickel-plating film.
Cross-Sectional STEM Observation and EDX-EELS Analysis of Corroded Areas
In order to examine the manner in which test piece (1) was corroded, cross-sectional STEM observation of the corroded areas was performed, and EDX-EELS analysis was also carried out to examine the chemical condition of the corroded areas of the chromium and nickel films.
To prepare the test sample, a focused ion beam (FIB) STEM processor was used to deposit a carbon CVD protective film on the target area. To enable its use as an observation specimen, 30-kV and 16-kV FIB beams were used to render the sample sufficiently thin for the TEM beam to pass through it.
JEM-ARM200F, manufactured by JEOL, was used for STEM observation. Observation was performed using an acceleration voltage of 200 kV in the STEM-HAADF imaging mode.
EDX analysis was carried out using a CENTURIO (100 [mm.sup.2]) detector, manufactured by JEOL, and a NORAN System7 analyzer, manufactured by ThermoFisher SCIENTIFIC.
EELS analysis was carried out using a Model 965 GIF Quantum ER1, manufactured by Gatan, with the analysis conditions set to an EELS aperture of 2.5 mm, a dispersion of 0.25 eV/ch, and a zero-loss peak FWHM of approx. 0.6 eV.
Figure 12 shows STEM cross-sectional views of the corroded areas of test piece (1). The areas analyzed by EELS are indicated by the red circles.
STEM cross-sectional observation of the corroded areas of test piece (1) showed that not only was the nickel-plating film corroded, but the chromium plating film was also corroded from the surface. In addition, it also showed that corrosion of the nickel film did not advance downward, but along the interface with the chromium layer.
The areas in Figure 12 where chromium and nickel had been corroded (circled areas) were analyzed using EDX-EELS to determine the chemical conditions. Figures 13 to 16 show the results.
The EDX analysis results in Figures 13 and 14, and the EELS spectrum in Figures 15 and 16 show the condition of the chromium and nickel oxides.
A peak extremely close to that of chromium oxide, Cr2O3, was detected in the corroded area of the chromium layer. In addition, NiO was detected in the corroded area of the nickel layer , , .
These results clearly demonstrate the presence of chromium and nickel oxides, indicating that both layers were corroded.
It has been clearly demonstrated that the presence of fulvic acid and calcium chloride in the soil has a large effect on the corrosion of chromium plating. The three following factors are believed to be involved in the corrosion mechanism:
* The cation exchange reaction of fulvic acid and calcium chloride decreases pH, promoting corrosion of the chromium film.
* Fulvic acid and chromium form a complex that destroys the chromium passivation film.
* Fulvic acid and chromium form a complex that changes the potentials of the chromium and nickel layers, causing the sacrificial corrosion protection effect of the nickel to disappear.
Based on the above discussion, verification was conducted with regard to the following: (1) change in pH, (2-1) formation of complex, (2-2) condition of chromium passivation film, and (3) disappearance of the sacrificial corrosion protection effect of nickel.
Verification of Change in pH due to Change in Concentration of Fulvic Acid
To verify whether the reaction of fulvic acid and calcium chloride produces a change in pH, fulvic acid was added to a saturated solution of calcium chloride to a concentration of 20 mass%, and the pH was measured.
A pH meter (HORIBA F-54) and pH electrode (9615-10D) were used to measure pH.
Figure 17 shows the results. The measurements show that as fulvic acid concentration increases, pH decreases. In the case of the mud made of a mixture of calcium chloride solution and kaolin in order to simulate the brine mud environment discussed earlier, the measured pH value was only reduced to pH3 or pH4. However, on addition of 11 mass% of fulvic acid, which was detected in Russian soil as in Figure 6, a value of pH1 was observed.
(2-1) Verification of Formation of Complex
Next, to verify whether fulvic acid and plated chromium form a complex, a mud test was conducted using Russian soil and kaolin. After the test, the removed soil and chromium plated surfaces were measured using XAFS.
The Toyota beamline (33XU) in the SPring-8 synchrotron radiation facility was used for measurement. Fluorescence using a four-element SDD detector was employed as the measuring method.
Figure 18 shows the results. Since changes in the condition of the chromium are expected to appear in the region near the rise of an absorption edge in the XAFS spectrum (XANES region), Figure 18 shows the spectrum in the XANES region.
In the measurements of Russian soil, which contains fulvic acid, peaks attributable to a chromium-fulvic acid complex were detected at 6010 eV and 6020 eV .
In addition to the peaks of the chromium-fulvic complex, a metallic chromium peak was detected at 5990 eV from the surface of the chromium plating film after the test.
This shows that the chromium plating was not simply corroded and dissolved, but that the fulvic acid contained in Russian soil and the chromium in the chromium plating formed a complex, and also that, although the chromium plating surface looked like corroded chromium after the corrosion test, not all of the chromium surface was corroded since part of it remained as metallic chromium.
(2-2) Verification of Condition of Chromium Passivation Film
In the preceding section, it was verified that a chromium-fulvic acid complex is formed. Next, it will be verified whether fulvic acid prevents regeneration of the chromium passivation film.
The chromium plating test piece referred to earlier was used to measure chromium, and, in order to measure nickel, the nickel layer of a chromium plating test piece was exposed by dissolving its chromium layer.
As pretreatment of the test piece, a 5% solution of Na2CO3 was employed as the electrolyte, and a potential of 1000 mV was used. When the electric current had decreased from several tens of mA down to a few [micro]A, the chromium layer was deemed to have completely dissolved.
The test pieces were masked, leaving a circular 10 mm diameter measuring area on the chromium and nickel surfaces. Two solutions were prepared: a saturated calcium chloride solution, and a solution made by adding 11 mass% of fulvic acid to a saturated calcium chloride solution. Using each of these solutions, measurements were carried out to obtain the potentials and polarization curves for the chromium and nickel test pieces.
Measurement was begun after the sample had been immersed for five minutes without degasification and with cathodic reduction. When using the solution without fulvic acid, measurement was begun at the potential 0.3 V lower than the natural electrode potential, and when using the solution with fulvic acid, at the potential 0.05 V lower. The scanning rate of the potential was 20 mV/min. A potentiostat HZ-300, manufactured by Hokuto Denko, was used as the polarization measuring instrument, a confidence value of up to 1 [micro]A being assumed.
Figure 19 shows the results. Without fulvic acid, there is a passivation range, while no passivation range is observed with the solution to which fulvic acid has been added. The difference between polarization curves shows that the combined presence of fulvic acid and calcium chloride disrupts recovery of the chromium passivation film. 
(3) Disappearance of the Sacrificial Corrosion Protection Effect of Nickel
Nickel exhibits a sacrificial corrosion protection effect when its potential is lower than that of chromium. In order to verify whether natural potential changes due to the presence of fulvic acid, causing this protection effect to disappear, changes in the natural potential of chromium and nickel were measured using saturated solutions of calcium chloride to which fulvic acid had been added at different concentrations up to 20 mass%. The same pretreatment was conducted and the same measurement instrument used as in the preceding section 5.3 (2-2).
Measurement of the natural potential was begun after the sample had been immersed for five minutes without degasification or cathodic reduction.
Figure 20 shows the results. When the concentration of fulvic acid was over 10 mass%, the difference between the potentials of chromium and nickel was almost zero. This shows that, as the concentration of fulvic acid increases, the sacrificial corrosion protection effect of nickel disappears. Since the concentration of fulvic acid in Russian soil is 11 mass%, it is probable that a similar phenomenon occurs in Russia.
In addition, when the concentration of fulvic acid was over 20 mass%, the difference between the chromium and nickel potentials remained approximately constant, without the nickel potential overtaking that of chromium.
Based on verifications (1) to (3), the mechanism of the specific type of corrosion of chromium plating that occurs in Russia is as follows.
First, the specific type of corrosion that occurs in decorative chromium plated parts when used in winter in Russia results from: the large fulvic acid content of Russian soil; calcium chloride, a deicing agent, being sprayed in large quantities; and the fulvic acid and calcium chloride mixing on the road to form mud which attaches to the outer panels and decorative chromium plated parts of vehicles for a long period.
The mechanism of corrosion of chromium plating depends on a combination of phenomena, i.e., fulvic acid reducing pH, fulvic acid and chromium forming a complex that disrupts the generation of the chromium passivation film, and the sacrificial corrosion protection effect of nickel disappearing due to a change in the difference between the chromium and nickel potentials.
Furthermore, from the data regarding the difference between the chromium and nickel potentials and the fact that metallic chromium was detected in the products of corrosion, it has become clear for the first time that the previously assumed corrosion mechanism, which assumes that corrosion occurs only through dissolution of the chromium surface, is not correct.
It is not the case that chromium is preferentially dissolved as the result of a complete reversal of the potential difference between chromium and nickel; instead fulvic acid causes the nickel potential to become almost the same as that of chromium, making it probable that nickel, with its only slightly lower potential, is corroded along the interface with the chromium layer. In addition, from the fact that metallic chromium was detected from the products of corrosion after the corrosion test, it is probable that corrosion of the nickel layer results in exfoliation of the upper chromium layer.
The mechanism of the specific type of corrosion that occurs in chromium plating in Russia can be summarized as follows:
1. Corrosion that starts at the surface of chromium layer
** The chromium in the chromium plating and fulvic acid in Russian soil form a chromium-fulvic complex, and, due to its characteristics, this complex adheres to the chromium surface, disrupting generation of the chromium passivation film, and consequently resulting in corrosion of the chromium surface.
** An ion exchange reaction inside the mud generates hydrochloric acid, which dissolves and corrodes the chromium.
2. Exfoliation of the chromium film due to corrosion of the nickel at the chromium-nickel interface
** As the concentration of fulvic acid increases, the potential of the nickel approaches that of chromium, and when the quantity of fulvic acid is over 10 mass%, the difference between the chromium and nickel potential becomes small. The sacrificial corrosion protection effect of the nickel disappears, and the nickel corrodes along the interface with the chromium, loss of the underlying nickel resulting in exfoliation of the chromium layer.
With regard to the specific type of corrosion of decorative chromium plating that occurs in Russia, the reasons for it being specific to Russia were made clear, and the mechanism of corrosion was determined by verifying the effect of the relevant factors on chromium plating corrosion.
Organic compounds in soil have been reported to have little influence on corrosion . However, this study has extended its investigations to fields other than corrosion, such as geology and pedology, discovering that the fulvic acid in Russian soil contributes to chromium plating film corrosion through its interaction with a component in de-icing agents.
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12. Corrosion Data Survey - Metal Section (1985)
Yuko Kajiyama, Toshikazu Obata, Tsuyoshi Sugimoto, Masahiro Nakamura, and Motohide Mori
Toyota Motor Corporation
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|Author:||Kajiyama, Yuko; Obata, Toshikazu; Sugimoto, Tsuyoshi; Nakamura, Masahiro; Mori, Motohide|
|Publication:||SAE International Journal of Materials and Manufacturing|
|Date:||Aug 1, 2016|
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