Non Chromate Coating For Al Alloy Biology Essay

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To inhibit corrosion on aluminum 2024, and prevent this pitting behavior from occurring, hexavalent chromates have been the gold standard for use in both conversion coatings and as coating pigments. However, hexavalent chromates have been shown to be extremely toxic and carcinogenic. Additionally, they are somewhat water soluble and have been shown to have entered the supply of drinking water in several areas.[9-14] Due to the dangers associated with the use of hexavalent chromates, they have come under ever increasing regulation necessitating the use of alternative corrosion inhibitors that do not have the hazards associated with hexavalent chromates.[15]

Some of the alternatives that have been explored include other inorganic inhibitors such as molybdate, vanadate, or cerium based inhibitors, as well as organic inhibitors that can in some way, either by covalent or intermolecular forces, be bound to a metal's surface.[16-19] Conductive polymers such as polypyrrole and polyanaline have also been investigated due to their environmentally friendly nature and promising performance.[20-22]

Another alternative to hexavalent chromium may be to use trivalent chromium based inhibitors. It has been shown that chromium(III) compounds are much less biologically active in comparison to chromium(VI) compounds. This is mostly due to the decreased uptake of chromium(III) into cells in comparison to chromium(VI).[23] Additionally, investigation have shown that chromium(III) is capable of inhibiting corrosion on aluminum alloys.[24, 25] However, the performance of Chromium(III) based inhibitors is often times inferior to that of hexavalent chromates. This study seeks to determine the effectiveness of a trivalent chromium pretreatment compared to a hexavalent chromate pretreatment with both a chromated and a nonchromated primer system and a MIL-DTL-64159 CARC top coat . The evaluation was done using the scanning vibrating electrode technique (SVET), electrochemical noise measurements (ENM) and X-ray photoelectron spectroscopy (XPS).

Hexavalent chromium based pretreatment

Alodine 1200S

The room temperature Alodine bath. pH = 1.5, consisted of 9.0 g/L a mixture of 50-60% CrO3, 20-30% KBF4, 10-15% K3Fe(CN)6, 5-10% K2ZrF6, and 5-10% NaF by weight which is ~ 0.065M in Cr.

Hexavalent chromium free pretreatments

Trivalent Chromium Pretreatment (TCP)

Alodine 5200 and Alodine 5700

Bi-K Aklimate

Boeing Boegel Sol gel

Brent Oxsilan AL-0500

MacDermid Chemidize 727ND

X-It PreKote

2. Experimental

2.1 SVET measurement

The corrosion activity of PAFC-coated AA 2024-T3 in the vicinity of a defect was measured using a SVET system (Applicable Electronics) employing a Pt-black deposited Pt-Ir microelectrode (Microprobe, Inc.) as the vibrating probe and a platinum wire as a quasi-reference electrode. The sample (1 cm2) was masked by polyester tape so as to form a scan area of 2.5 - 2.5 mm. An artificial defect was introduced in the center of the scan area. The probe made measurements on a 20 - 20 grid (requiring ca. 10 min). All the SVET measurements were performed under open circuit conditions in a cell containing ca. 5 mL 3 wt% NaCl.

The current density map associated with anodic and cathodic corrosion activities over the scanned area was obtained and displayed in 3D maps, showing the spatial distribution of the vertical component of the current density as a function of the (X, Y) position in the scan region. In these SVET maps, anodic currents are positive and cathodic currents are negative. A contour map of the current density is projected onto the bottom of the 3D map. In some cases, the data is also presented as current vectors superimposed onto an optical micrograph of the sample, showing both horizontal and vertical components of the current. Three replicate SVET measurements were made for each sample type. Results presented are representative of the set.

2.2 Electrochemical noise measurements (ENM)

Electrochemical noise measurements (ENM) were used to evaluate protective properties of different coating systems. ENM was measured using the traditional three-electrode configuration, as schematically shown in Fig.1. This configuration involved a reference electrode and two identical coated panels as working electrodes electrically connected via a salt bridge. Their coupled potential was measured with respect to a saturated calomel reference electrode. The potential and current noise was measured simultaneously by Gamry potentiostat in zero resistance ammeter (ZRA) mode with Gamry ESA400 software. Each measurement lasted 256 s with the data acquisition frequency of 5 Hz, and hence composed of 1280 points.

The electrolyte used in this measurement was 3% NaCl solution.

Fig.1. Schematic diagram of the experimental configuration for the electrochemical noise measurement.

The data were analyzed by ESA410 analysis software. Fig.2 shows examples of potential and current fluctuations acquired for the 1200 N sample. The standard deviations both for the potential fluctuations and for the current fluctuations are calculated over each 1280 point data.



where Vi, Ii are the (potential, current) data pairs, , the mean values of the recorded potential and current noise, respectively, and n the total number of data points. The noise resistance (Rn) is subsequently derived by Eq. (3).


Fig. 2. Examples of the potential/current fluctuations acquired for the 1200 N sample after 6d immersion in 3% NaCl solution

2.3 Electrochemical impedance spectroscopy

EIS measurements were carried out using a Gamry PC4/300 potentiostat/galvanostat (EIS300 software). The samples were masked by polyester tape, except for a 10 - 15 mm exposed surface as working area (1.5 cm2). The spectra were acquired at the corrosion potential Ecorr over the frequency range of 105 ~ 0.01 Hz at 10 points/decade using an AC signal amplitude of 10 mV RMS. All EIS measurements were performed in an enclosed three-electrode cell employing a SCE as the reference electrode. To simulate the low- or no-oxygen condition under the topcoat, the DHS was purged with N2 for 2 h before the measurement and throughout the experiments.

2.4 XPS analysis

For XPS analysis, the sample (1 cm2) was masked by polyester tape so as to form exposed area of 1-1 cm2. In the center of the exposed area, a 2 mm diameter hole was drilled just through the coating to expose the substrate (1-1 cm2). The sample was immersed in 3% NaCl for different periods. XPS analysis was done in the Ø 2 mm hole.

An artificial defect was introduced in the center of the scan area.

The XPS measurements were performed on an SSX-100 system (Surface Science Instruments) quipped with a monochromated Al Ka X-ray source, a hemispherical sector analyzer (HSA) and a resistive anode detector. The X-ray spot size was 1x1 mm2, which corresponded to an X-ray power of 200 W. Each survey spectrum was collected with 10 scans at 150 eV pass energy using 1 eV/step.

3. Results

3.1 SVET measurement

1200 N



Fig. 4 SVET current density maps for the PPy-inhibitor Al flake composite coatings at immersion of 24 h in DHS. The optical image (e) with current vectors superimposed shows the location of the defect on the coatings.

3.2 Electrochemical noise measurements

For the topcoated coating systems, the noise resistances (Rn) in the initial stages of immersion in 3% NaCl solution are compared in Fig. 3 for the three coating systems, chromate/MIL-PRF-23377C2/MIL-PRF-85285 (CCC/7C), chromate/MIL-PRF-23377N/MIL-PRF-85285 (CCC/7N) and TCP/MIL-PRF-23377N/MIL-PRF-85285 (TCP/7N).

The chromate/MIL-PRF-23377C2/MIL-PRF-85285 (1200-C2) coating system showed the best performance: Rn as high as 2-1010 Ω-cm2, with an average value ~1010 Ω-cm2. While the chromate/MIL-PRF-23377N/MIL-PRF-85285 (1200-N) coating system showed Rn in the order of 106 Ω-cm2. The MIL-PRF-23377C2 primer shows 4 orders of magnitude of improvement in the barrier property compared with the MIL-PRF-23377N primer. With the same MIL-PRF-23377N primer, the TCP pretreatment improved Rn by almost 2 orders of magnitude compared with the chromate pretreatment.

It is remarkable that the coating systems were differentiated and qualitatively ranked in relatively short periods of immersion.

Fig. 3. Comparison of noise resistances (Rn) measured in 3% NaCl solution for the three samples, CCC/MIL-PRF-23377C2/MIL-PRF-85285 (CCC/7C), CCC/MIL-PRF-23377N/MIL-PRF-85285 (CCC/7N) and TCP/MIL-PRF-23377N/MIL-PRF-85285 (TCP/7N)

It should be noted that an exact physical meaning of the noise resistance is still not clear, although it has been discussed for many years. From literature and experience, Rn could be assumed a measure of the diffusion barrier properties of the coating: Rn shows good agreement with the zero frequency limit of impedance modulus (polarization resistance Rp) and/or coating resistance R [19, 21, 22]. Typically, values for Rn equalor higher than 109Ω-cm2 are measured for excellent coatings. The literature on ENM use in coatings have indicated that, Rn (Ω-cm2)

Good to Excellent coatings:

Rn (Ω-cm2) 109 to greater than 1013 that maintain > 2 weeks,

Fair to Good coatings:

Rn (Ω-cm2) 106 to greater than 109 Ω -cm2 that maintain < 2 weeks,

Poor coatings:

Rn (Ω-cm2) less than106 Ω -cm2 that do not maintain even this level

The noise resistances (Rn) are compared in Fig. 4 for the five samples: CCC/23377C2 (7C for no topcoat and 7C/9 for with topcoat), TCP/23377N (TCP/7N for no topcoat and TCP/7N/9 for with topcoat), TCP/53022/64159 (TCP/2/9). As for the immersion for the series is just beginning, the Rn trend is not clear yet from the data obtained now. It may take longer immersion time to make it clear.

Fig. 4. Comparison of noise resistances (Rn) measured in 3% NaCl solution for the five samples: CCC/23377C2 (7C for no topcoat and 7C/9 for with topcoat), TCP/23377N (TCP/7N for no topcoat and TCP/7N/9 for with topcoat) and TCP/53022/64159 (TCP/2/9).

3.3 EIS

3.4 XPS analysis


Fig. 2. XPS in defects on the CCC and TCP coating (w/o topcoat) XPS spectrum of CCC/23377C2 and of TCP/23377N

Fig. 2. XPS in defects on the CCC and TCP coating (w/o topcoat) XPS spectrum of CCC/23377C2 and of TCP/23377N

4. Conclusion


The support of this research by the US Army Research Laboratory under grant No. W911NF-04-2-0029 is gratefully acknowledged.