Development of trivalent chromium passivation for Zn platng with high corrosion resistance after heating.
Trivalent chromium passivation is used after zinc plating for enhancing corrosion resistance of parts. In the passivating process, the amount of dissolved metal ions (for example zinc and iron) in the passivation solution increases the longer the solution is used. This results in a reduced corrosion resistance at elevated temperatures. Adding a top coat after this process improves the corrosion resistance but has an increased cost. To combat this, we strove to clarify the mechanism of decreased corrosion resistance and to develop a trivalent chromium passivation with a higher corrosion resistance at elevated temperatures.
At first, we found that in parts produced from an older solution, the passivation layer has cracks which are not seen in parts from a fresh/new solution. These cracks grow when heated at temperatures over 120 degrees Celsius.
Next we researched the reason for cracks to occur and found that the main difference between an old and new solution's layer is the amount metal deposits in it. These metal ions deposit into the passivation as hydroxides, and the larger the quantity in this layer the more the layer contracts by heating, meaning the newer the solution the less the layer contracts.
So, we investigated developing a new solution to improve the corrosion resistance after heating through the reduction of metal ion deposits in the passivation layer. We achieved this reduction by adding organic carboxylic acid to chelate the dissolved metal ions. The carboxylic acid prevents excess depositing of these ions in the passivation layer. Using this developed solution, cracks disappeared and the corrosion resistance after heating was improved.
CITATION: Kawaguchi, H., Funatsumaru, O., Sugawara, H., Sumiya, H. et al., "Development of Trivalent Chromium Passivation for Zn Platng with High Corrosion Resistance after Heating," SAE Int. J. Mater. Manf. 9(3):2016.
Zinc plating is applied for parts that need corrosion resistance with low price. Generally, Trivalent chromium passivation is applied with zinc plating at same time to prevent the zinc corrosion (white rust). Sometimes, the specifications for these coatings (zinc plating + passivation) includes corrosion resistance after heating at 120 degrees Celsius for 24 hours. It is difficult to meet this requirement using only these coatings.
The main chemical reaction of the trivalent chromium passivation has two steps. At first, zinc dissolves and pH goes up. Then the metal ions (for example chromium) deposit as the passivation layer.
Zn+2[H.sup.+][right arrow][Zn.sup.2+]+[H.sub.2][up arrow] (1)
[Cr.sup.3+]+3[OH.sup.-][right arrow]Cr[(OH).sub.3][right arrow][Cr.sub.2][O.sub.3] (2)
Therefore, in the production line, metal ions such as zinc and iron are increasing in the passivation solution by dissolving from the parts' surface. Zinc is dissolved from plating surface, and iron is dissolved from the base metal surface at non-plated areas such as inner face of pipes. The trivalent chromium solution in which these metal ions are dissolved over time is referred to in this report as the "long run" solution. Figure 1 shows the neutral salt spray test result (72 hours) of samples passivated in fresh/new solution and in long run solution after heating at 120 degrees Celsius for 24 hours. The zinc concentration is 8.9 g/L and iron concentration is 26 mg/L in the long run solution. The result for the part passivated in new solution is good (no rust), but the result for the part passivated in the long run solution is not good (white rust). Thus the deterioration of the corrosion resistance after high temperature exposure may be effected by the dissolved ions.
Adding a top coat after this process improves the corrosion resistance but has an increased cost. To combat this, we strove to develop a trivalent chromium passivation with a higher corrosion resistance at elevated temperatures.
Mechanism of the Corrosion Resistance Reduction after Heating
We investigate the reason for the corrosion resistance reduction after heating before developing new passivation. Test samples (steel plates and steel pipes) are passivated by the condition shown in Table 1 after zinc plating (about 10 micrometer). Some of the samples are then heated to 120 degrees Celsius for 24 hours.
The test samples with and without heating were then evaluated using the following methods.
* Neutral salt spray test for 72 hours
* Surface observation by scanning electron microscopy, SEM
* Cross-sectional observation by Transmission electron microscopy, TEM
* In-depth composition analysis by X-ray photoelectron spectroscopy, XPS
The NSS test result is shown in Figure 2. Both samples treated in the new solution (with/without heating) have no white rust and good corrosion resistance. The sample treated in the long run solution without heating also has no white rust, but the sample with heating has white rust. This shows that the corrosion resistance decreases after exposure to high temperatures. We verified the passivation's condition by SEM (see Figure 3). The new solution's passivations have no damage. The long run solution's part has small cracks, and these cracks grow by heating. The cracks growth is caused by the passivation's constriction with high temperature. The crack growth may cause the passivation to peel off from the zinc surface. Thus the corrosion resistance can be decreased by heating.
Next, we investigate the reason for the cracks. The trivalent chromium passivation is generated by deposit of the metal ions in the solution with elevated pH. In the long run solution, the dissolved ions (zing and iron) concentration in the passivation layer is preseumed to be higher than the new solution's. We investigated the passivations' makeup and the dissolved ions' distribution by cross-sectional TEM observation and by XPS analysis. Figure 4 shows the cross-sectional TEM observation result. The new solution's passivation can be seen as a uniform single layer, but the long run solution's passivation can be seen as a thicker double layer. XPS analysis was used to identify the double layer(shown in Figure 5). From the XPS result, the trivalent chromium passivation consists of a chromium layer and an interlayer with a high concentration of zinc. The interlayer of the long run solution's passivation is thicker than the new solution's interlayer, and there are iron deposits in the chromium layer.
We clarify the effect of zinc and iron by evaluating the samples made with the conditions shown in table 2. Figure 6 shows the NSS test result for the heated samples. Either metal drops the corrosion resistance after heating. Figure 7 shows the surface observation result. Both zinc and iron cause cracks to form. The passivation layer has peeled off in some areas - it is easy to imagine these samples will have lower corrosion resistance. Figure 8 shows the XPS results for these samples. From this it can be observed that each ion affects the passivation layer independently, as zinc increases the interlayer and iron deposits in the chromium layer. From these results, the corrosion resistance is reduced after exposure to high temperatures due to crack growth, caused by dissolved metal ions (Figure 9). The dissolved metal ions may deposit as hydroxides, which contain water molecules. The cracks may be generated by the hydroxides dumping water molecules at high temperature. Zinc dissolved at the beginning of the passivation reaction may deposit between the zinc plating and the chromium layer as the interlayer. This interlayer may generate easily at areas where the passivation solution cannot be mixed enough.
Improvement of the Corrosion Resistance after Heating
Ideas for improving the corrosion resistance after heating are shown in Table 3. Adding top coating is the normal action. However, to avoid the increased cost of top coating, we sought a method to prevent the depositing of the dissolved metal ions. O ne method would be to use a chelating resin that can remove the dissolved ions, but this would also increase cost (due to the addition of additional equipment) and cause the removal of chromium ions at the same time. The option we selected was adding a chelating agent into the passivation solution to stabilize the dissolved ions in the solution. Or ganic carboxylic acid, which is in the normal passivation solution, is selected for the chelating agent in consideration of the deposit of the passivation layer.
Cross-sectional TEM observation and the XPS analysis are used (see Figure 10 and 11) to check the organic carboxylic acid's effect. The effect on zinc can be seen in the interlayer's thinning in Figure 10 and 11. The effect on iron can be seen the iron concentration in the chromium layer in Figure 11. Finally, it can be seen that the passivation layer's construction is almost same as the new solution's passivation. Surface observation (see Figure 12) shows that the adding the organic carboxylic acid decreases the heating damage in the passivation layer. Figure 13 shows the NSS test result. After 72 hours in the NSS test chamber, no rust generates on the parts with/without heating, indicating we can get a good passivation with high corrosion resistance after heating.
Controlling Proper Concentration of the Organic Carboxylic Acid
In the previous chapter, adding the proper amount of organic carboxylic acid improves the corrosion resistance after heating. However, excess amounts of organic carboxylic acid may be detrimental to quality. We verified the effect of excessive additions of carboxylic acid. Figure 14 shows the difference of appearance and corrosion resistance after heating with different organic carboxylic acid amounts. Considering the appearance, excess organic carboxylic acid causes tarnish of the passivation surface. The corrosion resistance of the tarnish surface parts decreases. This tarnish may occur in the long run solution and the bad agitated solution. The reason for the tarnish is presumed to be a thick interlayer caused by a high concentration of zinc on the reaction surface.
Figure 15 shows the surface observation and the XPS analysis result. From the surface observation, the excess organic carboxylic acid parts has cracks after heating. The interlayer thickness grows in the excess organic carboxylic acid parts, but iron does not deposit. Thus the reason for the tarnish is determined to be the thicker interlayer. The excess organic carboxylic acid may function as a buffer of the pH and inhibit the pH change. As a result, the first reaction of passivation (dissolving zinc plating) may be accelerated.
From this result, we determined that the organic carboxylic acid addition can stabilize the dissolving ions, but it can also accelerate the dissolving reaction and cause the zinc concentration near the zinc surface to be too high to chelate the dissolved ions. Thus excess organic carboxylic acid addition leads to lower corrosion resistance after heating because of the thicker interlayer. Additionally, accelerating the dissolving reaction shortens the solution's life and increases the cost of the passivation process. Therefore the controlling the carboxylic acid's concentration is important. We developed a new additive solution to control the concentration.
The increase of the concentration of dissolved ions in the passivation solution over time leads to the deposit of these ions in the passivation layer, causing the corrosion resistance after heating to decrease by generating cracks. To improve the corrosion resistance, adding the organic carboxylic acid is effective. This technology allows for the following two cost reduction ideas.
1. Eliminate top coating
The extra cost of the top coating solution is not needed when using this passivation.
2. Apply baking process for hydrogen embrittlement relief after passivation
Normally the baking process for hydrogen embrittlement relief is done before the passivation process, but this new technology allows the baking process to be done after the passivation process. This allows the parts to be zinc plated and passivated in one process. As a result, loading and unloading processes are reduced and the postprocessing is not necessary.
DENSO CORPORATION has been using the passivation process developed by this technology in-house in plants in Japan from November 2012.
Hiroshi Kawaguchi, DENSO CORPORATION
Work phone: +81-566-25-7779
Ryuta Kashio, DIPSOL CHEMICAL Inc.
Work phone: +81-50-5846-3002
Hiroshi Kawaguchi, Osamu Funatsumaru, Hiroyoshi Sugawara, Hiroshi Sumiya, and Takanobu Iwade
Tomitaka Yamamoto, Takashi Koike, and Ryuta Kashio
DIPSOL Chemical Inc.
Table 1. Test condition for analyzing the long-run effect New solution Long run solution [Cr.sup.3+] 4.2 g/L 4.2 g/L Dissolved [Zn.sup.2+] 0 g/L 20 g/L ion [Fe.sup.3+] 0 mg/L 100 mg/L Temperature 30[degrees]C pH 2.0 Time 40 sec Table 2. Test condition for analyzing the dissolving ions' effect New Long run solution solution Zn only Fe only Mixed [Cr.sup.3+] 4.2 g/L 4.2 g/L 4.2 g/L 4.2 g/L Dissolved [Zn.sup.2+] 0 g/L 20 g/L 0 g/L 20 g/L ion [Fe.sup.3+] 0 mg/L 0 mg/L 100 mg/L 100 mg/L Temperature 30[degrees]C pH 2.0 Time 40 sec Table 4. Test condition for analyzing the organic carboxylic acids' effect Long run solution added organic carboxylic acid [Cr.sup.3+] 4.2 g/L Dissolved [Zn.sup.2+] 20 g/L ion Fe[.sup.3+] 100 mg/L organic carboxylic Proper quantity acid (for dissolved ion) Temperature 30[degrees]C pH 2.0 Time 40 sec Table 5. Test condition for analyzing the organic carboxylic acids' effect (for excess quantity) Long run Long run solution added solution organic carboxylic acid [Cr.sup.3+] 4.2 g/L 4.2 g/L 4.2 g/L Dissolved [Zn.sup.2+] 20 g/L 20 g/L 20 g/L ion [Fe.sup.3+] 100 mg/L 100 mg/L 100 mg/L organic carboxylic none Proper excess acid (Tor dissolved ion) quantity quantity Temperature 30[degrees]C pH 2.0 Time 40 sec
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|Author:||Kawaguchi, Hiroshi; Funatsumaru, Osamu; Sugawara, Hiroyoshi; Sumiya, Hiroshi; Iwade, Takanobu; Yamam|
|Publication:||SAE International Journal of Materials and Manufacturing|
|Date:||Aug 1, 2016|
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