Enhancement of Acid-Chloride Resistance in a Chromate Conversion

19 Enhancement of Acid-Chloride Resistance in a Chromate Conversion Coating V. S. Agarwala

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: October 14, 1986 | doi: 10.1021/bk-1986-0322.ch019

Naval Air Development Center, Warminster, PA 18974

Chromate conversion coatings perform poorly in environments containing acidified chloride. In salt/S0 spray tests the substrate metal is heavily pitted after three to four days of exposure. In this work, a new coating was developed which improved the corrosion resistance of the conventional chromate coating remarkably. The new coating protected the substrate metal (7075-T6 A1) in 5% NaCl + SO spray for up to two weeks. It even extended the stress corrosion cracking resistance of 7075-T6 A1 alloy. Studies showed the leaching rate of Cr through the new coating was significantly reduced. The technique of developing the coating consisted of incorporating molybdate into the pre-existing coating containing chromium oxides on an aluminum surface. 2

2

On aluminum surfaces, chromate conversion coatings are mostly used to protect the metal from corrosion both as a pretreatment for bonding primers and paints, and for bare metal corrosion( 1-2 ). On s t e e l s , they are applied after the surfaces have been plated with cadmium. For most m i l i t a r y hardware (naval a i r c r a f t systems), use of chromate conversion coatings are standard practice and are s p e c i f i e d (Qualified Product L i s t , MIL-C-81706) for corrosion protection from i n d u s t r i a l and marine (naval) environments. Although chromate conversion coatings offer good resistance to most ambient environments, including up to 5% NaCl, they f a i l to perform when the environments become a c i d i c . Aboard a i r c r a f t c a r r i e r s , the stack gases and a i r c r a f t exhausts introduce large amounts of SO2, soot, and other carbonaceous products which are highly corrosive into the environment. This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

Dickie and Floyd; Polymeric Materials for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


POLYMERIC MATERIALS FOR CORROSION CONTROL

Recent studies on monitoring of a c a r r i e r environment ( _3 ) have shown that i t s corrosion severity p a r a l l e l s to a laboratory c y c l i c environment of 5% NaCl spray (fog) with 30ppm of SO2· The development of a new chemical conversion coating which i s also r e s i s t a n t to t h i s environment would be of great importance. Almost a l l aluminum structures are painted with organic polymers for corrosion protection. The purpose of incorporating an i n h i b i t o r interface (chromate conversion coating) between the substrate and the paint f i l m i s to ensure protection when paints f a i l to perform( £ ). I t has been generally accepted that no matter what kind of paint system, and how well i t i s applied, i t always w i l l have some porosity defects and w i l l degrade with time during service. In m i l i t a r y aerospace vehicles, the service conditions are most severe for paints. One way to increase protecion of aluminum i n a c i d i c marine environments i s to enhance the corrosion i n h i b i t i n g a b i l i t y of the inorganic coating. This paper describes the r e s u l t s of such an e f f o r t . A new coating which not only i n h i b i t s the attack of chloride ions but also that of the a c i d i c environments such as SO2 has been developed. Experimental Panels of high strength aluminum a l l o y (7075-T6) were used i n t h i s study. The panels were approximately 10 χ 3 χ 0.032 inch (25 χ 7.5 χ 0.08 cm) i n s i z e . The test environments for coating evaluation were: (1) a 5% NaCl spray (fog) chamber according to ASTM Standard Method of S a l t Spray (Fog) Testing (B117-73), and (2) a modified 5% NaCl/S02 spray (fog) chamber with SO gas introduced p e r i o d i c a l l y - ASTM Standard Practice for Modified S a l t Spray (Fog) Testing (G85-84(A4)). In the l a t t e r case, a constant spray of 5% NaCl was maintained i n the chamber and SO2 was introduced for one hour four times a day (every 6 hours)( 5^ ). Coated t e s t panels were examined for corrosion a f t e r one- and two-week exposure periods. The procedure for applying coatings was almost the same as used for conventional chromate conversion coating. The new conversion coating c a l l e d CMT was an a d d i t i o n a l surface a p p l i c a t i o n a f t e r standard chromate conversion coating. The experimental protocol used was as follows: 1. Cleaned i n non-etching mild a l k a l i n e solution at 160°F for ten minutes. 2. Double rinsed i n tap water. 3. Deoxidized i n a 15% H2SO4 + 2.5% chromic acid mixture at about 150°F for 2 to 5 minutes. 4. Double rinsed i n tap water and immersed i n a chromate conversion coating bath (MIL-C-81706) for f i v e minutes. 5. Washed i n tap water and rinsed i n d i s t i l l e d water. 6. Applied CMT (molybdate) treatment, rinsed i n tap water and d r i e d . CMT treatment i s a proprietary process developed at the Naval A i r Development Center. I t b a s i c a l l y incorporates molybdates i n the coating containing chromâtes.



AGARWALA

Enhancement of Acid-Chloride Resistance

A l l coatings were allowed to age for at least one week i n ambient a i r before testing f o r corrosion resistance. The CMT coated panels were compared with the standard chromate conversion coated panels for their performance. The evaluation plan consisted of the following measurements: (1) v i s u a l observations; (2) weight loss; (3) electrochemical p o l a r i z a t i o n ; (4) stress corrosion cracking; (5) scanning electron microscopy; and (6) XPS surface analysis. V i s u a l inspections of the specimens were made during and a f t e r the s a l t spray chamber exposure. They were inspected f o r general corrosion and p i t s . Conventional electrochemical measurements were made to determine their corrosion potentials and p o t e n t i a l vs. current density (E vs Log i) relationships. In both cases, neutral and a c i d i f i e d s a l t solutions were used; the d e t a i l s of the procedure are described i n ASTM Standard Practice for Conventions Applicable to Electrochemical Measurements i n Corrosion Testing (G3-74). Stress corrosion cracking tests were performed on 1/8 inch (3mm) diameter t e n s i l e specimens fabricated from 7075-T6 A l a l l o y . Proving rings were used as sustained load devices (ASTM Standard Practice f o r Preparation and Use of Direct Tension Stress Corrosion Test Specimens, G49-76). A l l t e n s i l e specimens were tested with their respective chemical conversion coating. Tensile loads of 40 k s i (275 MPa) were applied on each specimen. The neck region of the t e n s i l e specimen was surrounded by a p l a s t i c wrap containing cotton soaked with 1% NaCl. In order to accelerate the test environment, a c i d i f i e d (pH 2) 1% NaCl solution was used. These sustained load tests were c a r r i e d out for up to 300 hours. Results S a l t Spray Test : Panels coated with the standard chromate conversion coating and CMT were compared with each other i n their corrosion r e s i s t a n t properties i n several ways. The conventional 5% NaCl/SC>2 fog chamber tests showed excessive corrosion and p i t t i n g within one week on chromate conversion coated (CGC) 7075-T6 A l a l l o y panels. The CMT coated panels were almost uncorroded and without any p i t s . The plates i n Figure 1 show the conditions of the panels a f t e r 7 and 14 days' exposure i n t h i s environment. Even a f t e r 14 days' exposure the CMT panels were s t i l l f a r better than CCC panels. Weight Loss : A quantitative evaluation of the coatings, both CGC and CMT, was also made. The panels exposed i n salt/SC>2 fog chamber were measured and weighed before and a f t e r the test to calculate corrosion losses. A l l exposed panels were cleaned i n 1:1 HNO3 f o r after-the-test weight according to ASTM Standard Practice f o r Preparation, Cleaning, and Evaluating Corrosion Test Specimens (Gl-81, para 7.2.2). This i s to remove the corrosion products f o r c a l c u l a t i n g weight losses. The r e s u l t s of these measurements are given i n Table 1. Each panel was also examined v i s u a l l y and the


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2

number of p i t s were counted i n a s p e c i f i e d area, i . e . , 50cm surface area; an average of 4 specimens f o r each coating was determined and reported i n Table 1. Both the weight loss and p i t t i n g data f o r CMT showed s i g n i f i c a n t inprovement over the OCC. In fact, the p i t t i n g resistance for the CMT coating was excellent. In a 14-day period, i t showed only 10 p i t s / c m compared to 75 pits/cm for the OCC. 2

2

Table 1 - Corrosion test results of coated 7075-T6 A l a l l o y exposed to salt/S02 fog environment.

Coating

2

*Weight Loss, mg/cm /48 hrs.

# Pits/cm (in 14 days)


2

Bare

1.10 + 0.20

Heavily Corroded

CCC

0.65 ± 0.10

75 ± 20

CMT

0.25 + 0.05

10+3

* Based on specimen s i z e of 8 χ 6 χ 0.3cm

Stress Corrosion Cracking : Results of the proving ring tests of the coated specimens i n both the neutral and a c i d i f i e d s a l t solutions are given i n Table 2. Although there i s no s i g n i f i c a n t difference i n the f a i l u r e times f o r both the coated specimens i n neutral 1% NaCl, the differences were remarkable at low pH. The CMT coating was able to extend the stress corrosion cracking resistance of the chromate coated material from 60 to 90 hours at pH 2.

Table 2 - Stress corrosion cracking properties of coated 7075-T6 A l a l l o y from proving ring tests (stress = 40 k s i or 275 MPa).

Coating

*Time to F a i l u r e , Hours 1% NaCl 1% NaCl(pH 2)

CCC

300

60 ± 15

CMT

300

90 ± 15

*

Mean average of 10 specimens.



AGARWALA


Electrochemical Behavior : The corrosion (open c i r c u i t ) p o t e n t i a l values for the coatings were as given i n Table 3· For CMT coating, the corrosion p o t e n t i a l was l e a s t a c t i v e . Generally a less active (less negative) p o t e n t i a l indicates more corrosion r e s i s t a n t properties of the surface. The s t a b i l i t y of the CMT coating was excellent since i t showed no change in i t s corrosion p o t e n t i a l (-0.700V) as the environment was changed from neutral to a c i d i c or high to low chloride concentrations (cf. Table 3). The differences i n the corrosion resistant properties of the CQC and CMT coatings were better characterized from the p o t e n t i o s t a t i c anodic and cathodic p o l a r i z a t i o n behaviors i n a c i d i c s a l t solutions. The t y p i c a l p l o t s of the p o t e n t i a l vs current density are shown i n Figure 2. The curves i n Figure 2 also include a p l o t for untreated (bare) 7075-T6 aluminum a l l o y for comparison. Although both the coatings, CCC and CMT, showed s i g n i f i c a n t anodic s h i f t s i n the anodic p o l a r i z a t i o n curves when compared to untreated specimens, they were almost the same among themselves. The s h i f t s i n the cathodic p o l a r i z a t i o n curves (cf. Figure 2) between the CMT and CCC were highly s i g n i f i c a n t . The cathodic current densities for the CMT coated material were almost an order of magnitude lower than for CCC coated material. In other words, the corrosion rate (determined as the point of intersection of the anodic and cathodic T a f e l slopes) for the CMT w i l l be lower by an order of magnitude than f o r CCC i n 1% NaCl solution of pH 2. In 3.5% NaCl solution (pH 2), the p o l a r i z a t i o n p l o t s were almost the same but indicated an even greater s h i f t of the cathodic p o l a r i z a t i o n curve toward lower current densities for the CMT coating.

Table 3 - E f f e c t of chemical conversion coatings on corrosion potentials of 7075-T6 A l a l l o y .

Ecorr. vs SCE, Bare

CCC

volt

Environment

pH

CMT

3.5% NaCl

6

-0.725

-0.700

-0.700

3.5%

NaCl

2

-0.750

-0.720

-0.700

1.0%

NaCl

2

-0.760

-0.720

-0.700

Coating S t a b i l i t y : The s t a b i l i t y of a chemical conversion coating i s best described by the non-leaching character of i t s corrosion i n h i b i t i n g constituents and the insoluble nature of the oxides of the substrate metal. Chromate conversion coatings suffer from the lack of these properties. Thus, a study was conducted i n which a l l the coated panels


P O L Y M E R I C M A T E R I A L S FOR CORROSION C O N T R O L


216

-I Z

'

Ι Ο "

6

'

ι

» I I I 1 11 Ι Ο " *

5

CURRENT

I

1 1 I I I I I Ι Ο * "

DENSITY,

I

1

I

I I I I I 1 I O "

4

Amfr/cm

I

I

I

I I I I I I

3

2

Figure 2. Electrochemical p o l a r i z a t i o n behavior of chemical conversion coatings on 7075-T6 aluminum a l l o y i n 1% NaCl solution at pH 2.


10'

AGARWALA


were immersed i n 3.5% NaCl solutions of d i f f e r e n t pH for various exposure periods. A chemical analysis of the solutions by atomic absorption spectroscopy (AAS) showed that the amount of Cr leached out from the CMT coatings was approximately 1/3 of that from CCC. In a c i d i f i e d s a l t solutions (pH 2), leaching of Cr was much greater for CGC. Based on 14 days exposure i n 3.5% NaCl (pH 2), the OCC coating l o s t approximately 2.0 pg /cm of Cr compared to 0.4 pg/cm for CMT. The i n i t i a l chromium concentration i n both the CCC and CMT coatings was approximately 7-8 pg/cm . This was determined by stripping the coatings with 1:1 HN0 and analyzing the solution by AAS. 1

2

2

2


3

Surface Characterization : The morphology of the coating surface was examined by scanning electron microscopy and the composition of the films were determined form X-ray photoelectron microscopic (XPS) scans. Both the CGC and CMT coatings were a l i k e i n appearance but d i f f e r e d i n their chemical contents. Well pronounced peaks for Mo and Cr were found i n the CMT coating during energy dispersive X-ray analysis. Of course, there was no Mo detected i n the CCC coating. Elemental d i s t r i b u t i o n of Mo and Cr i n the CMT was very uniform. The surface composition of coatings, determined by XPS, was as follows: Coating

Cr

Mo

CCC

Cr203

—

CMT

CrOOH

M0O4"

Others M 0 , M (OH) X

"

Y

ΜχΟ

γ

These results indicate the presence of Cr(III) i n both the coatings but a less soluble hydrated oxide (CrOOH) i n CMT. Presence of Cr(VI) was not detected by the XPS technique. This was in agreement with the findings of Glass { §_ ), and Matienzo and Holub ( 1_ ). In the CMT coating, the presence of Mo (VI) as molybdate was s i g n i f i c a n t . I t was also noted that the OCC had at least two d i f f e r e n t forms of other metal (M) oxides ,probably aluminum, while the CMT had only one. Although no quantitative estimation of the individual species present was made, the approximation was that Cr and Mo concentrations were not s i g n i f i c a n t l y d i f f e r e n t i n the CMT coating. Discussion The mechanism of developing corrosion protective properties in an inorganic coating p r i n c i p a l l y consists of forming insoluble oxides on the metal surface. Additionally, oxides must have c e r t a i n corrosion i n h i b i t i o n (redox) properties which can protect the metal substrate from corrosive species l i k e CI" and SO4"." In the case of chromate conversion coating, CCC, the oxides of aluminum and chromium have been responsible for their corrosion i n h i b i t i v e properties which were derived from t h e i r soluble and insoluble portions of the




218

chromate f i l m . In acid-chloride environments, the protective properties of chromate become limited because the oxides of aluminum which hold the chromium oxides i n place are no longer insoluble. In t h i s study, these considerations were taken into account to a l t e r the composition of the surface f i l m s . In the CMT process, the chromate conversion coating was re-inforced with other substances which contained molybdates. This surface f i l m was f i v e times more stable than the conventional chromate coating i n both the neutral and acid-chloride environments. XPS studies and those by other investigators ( T_ ) suggest that chromium in chromate coatings i s mostly present as (^203. In the CMT coating i t was found to be i n CrOOH form. The hydrated oxides of chromium are known to be more stable than 0^03 ; they tend to form polymer type l i n k s ( 8_ ) and most probably protect the substrate metal by forming a b a r r i e r . A d d i t i o n a l l y Mo i n the CMT coating, which i s present as M0O4"" , i s also capable of forming polymeric l i n k s on the surface ( 9_ ). The presence of Mo(VI) i n the f i l m enhances the protection by counteracting the aggresive nature of C I " ions. Most probably Mo(VI) t i e s up CI" ions during the process. XPS analysis showed a very even d i s t r i b u t i o n of Cr and Mo oxides with very l i t t l e aluminum oxide on the top layers of the f i l m . In contrast, the CCC coating contained a s i g n i f i c a n t amount of both aluminum and chromium oxides. The surfaces which contain more stable and insoluble oxides can o f f e r better resistance to acid-chloride environments. The high corrosion r e s i s t a n t behavior of the CMT coating supports t h i s conclusion. Electrochemical studies showed a marked reduction i n the cathodic p o l a r i z a t i o n behavior which means the CMT coating o f f e r s a higher resistance b a r r i e r (potential drop) than the OCC to achieve the same cathodic reaction rates (current d e n s i t i e s ) . Superiority of the new coating was exhibited i n i t s a b i l i t y to reduce stress corrosion cracking s u s c e p t i b i l i t y of 7075-T6 A l a l l o y i n acid-chloride environments. Conclusions The conventional chromate conversion coating i s non-protective i n acid-chloride environments. A new coating c a l l e d CMT which contains oxides of Mo and Cr, was developed for aluminum A l l o y s . An evaluation of t h i s coating was made with the following conclusions: (1) In 5% NaCl + SO2 spray there was no corrosion of the substrate metal for up to two weeks; (2) CMT enhanced the stress corrosion cracking resistance of 7075-T6 A l a l l o y s i g n i f i c a n t l y ; (3) the CMT coating was more stable than CCC and showed almost no leaching of Cr or Mo i n acid-chloride environments; and (4) The composition of the CMT f i l m was mostly Mo as MoOj" and Cr as CrOOH, and had almost no oxides of aluminum on the surface.


AGARWALA


Acknowledgments The author thanks the assistance of the co-op students and P. J . Sabatini of Naval A i r Development Center (NADC); L. J . Matienzo of Martin-Marietta Corporation i n performing XPS analysis; and support of the NADC Independent Research program for f i n a n c i a l support.

Literature Cited 1.


2. 3. 4. 5. 6.

7. 8. 9.

Wernick, S. and Pinner, R. In "Surface Treatment of Aluminum"; Draper, Robert, Ed.; Teddington, U. Κ., 1972; Vol. I, pp. 233-290. . Katzman, Η. Α . , Malouf, G. M. and Stupian, G. W., Applications Surf. Sci. , 1979, 2, 416-432. Agarwala, V. S. In "Atmospheric Corrosion" Ailor, W. H . , Ed.; Wiley Interscience, New York, N.Y., 1982; pp. 183-192. Montle, J. F. and Hasser, M.D., Materials Performance , 1976, 50, 15-18. Ketcham, S. J . and Jankowsky, E. J., "ASTM Symposium on Laboratory Corrosion Tests and Standards", Bal Harbor, FL, 14-17 Nov. 1983. Glass, A. L., " A Radiochemical Investigation of the Leaching of Cr from Chemically Chromated A1 Alloy Surfaces, Part III, The "Uptake" of the Chloride Ion Into Such Surfaces," Naval Air Development Center, Warminster, PA; Report No. NADC-MA-6702, August 1967. Matienzo, L. J. and Holub, K. J., Applications Surf. Sci. , 1981, 9, 47-73. Lollar, R. M. In "Chromium-Chemistry of Chromium and Its Compounds"; Udy, M. J., Ed.; Reinhold Publishing, New York, N.Y., 1956; Vol. I, p. 306. Cotton, F. A. and Wilkinson, G. "Advanced Inorganic Chemistry"; Wiley Interscience, New York, N.Y., 1972; pp. 965-972.

RECEIVED February 24, 1986


Enhancement of Acid-Chloride Resistance in a Chromate Conversion

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