Non-Chromated Rinses as Replacements for Chromated Rinses

Article From: Products Finishing, , from Rock Island Arsenal

Posted on: 2/1/2000

Several non-chromated rinses were evaluated as replacements for chromated rinse solutions used in conjunction with zinc and manganese phosphate coatings applied per DOD-P-16323F...

Chromic acid rinses are specified in DOD-P-16232F, Military Specification, phosphate coatings, heavy manganese or zinc base (for ferrous metals) classes 1, 2 and 3. There are no allowances for non-chromium rinses in this specification, thus mandating the use of hexavalent chromium. These finishes are used on 75-90% of all small arms weapons parts.

Chromium rinses are also recommended for industrial processes using TT-C-490C, Federal Specification, cleaning methods for ferrous surfaces and pretreatments for organic coatings Type I. This is the steel pretreatment for application of paints, including the Army's Chemical Agent Resistant Coating. Although chromic acid rinses are normally used for TT-C-490C coatings, non-chromate rinses are allowed if the adhesion and corrosion resistance requirements are met. To help initiate Hazardous Waste Minimization Executive Order 12856, Section 3-313 on Community Right to Know and Pollution Prevention, which includes hexavalent chromium, selected commercially available non-chromate final rinses over heavy zinc-phosphate coatings were evaluated. This report compares non-chromate rinses versus the standard chromate final rinse over low-alloy steels with heavy zinc-phosphate coatings.

Experimental procedure

Five batches of 6 x 4 x 0.032-inch low-alloy steel laboratory panels were delivered to the plating shop. Lab personnel wiped them with acetone and then glass-bead blasted them at 35-45 psi using 70-140 mesh blast medium. The non-chromate rinses were prepared in the laboratory and transported to the plating shop approximately 3 hours before the tests were run to allow the solutions time to attain equilibrium. Six liters of each rinse were prepared. All the test panels were zinc phosphated for 15 min at 180F. Coating "X" was used in the zinc phosphate tank and controlled to produce coatings in accordance with DOD-P-16232F. Tank chemistries are listed in Table I for the different trials. After phosphating and rinsing, the test panels were transported to their respective final rinses without drying. All the panels could not be transported and rinsed simultaneously, therefore a portion of the test panels were removed from the plating rack and final rinsed while the remaining panels were left in the water rinse tank to prevent drying.

 


TABLE I—Tank Chemistry/Coating Weight
Zinc Phosphate Solution Chromic Acid Rinse


Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
Total Acid
24.47
26.59
25.14
20.69
21.40
Free Acid
5.90
5.93
5.80
4.22
4.16
Total/Free Ratio
4.15
4.48
4.33
4.90
5.14
Total Acid
2.66
3.30
3.10
2.12
2.86
Free Acid
0.33
0.75
0.58
0.29
0.23
Total/Free Ratio
8.06
4.40
5.34
7.31
12.40
Coating Weight (g/sq meter)
23.10
16.10
14.80
16.00
16.60

 


Six commercially available final rinses (A-F), an experimental final rinse solution made of molybdic/phosphoric acid (H) and the typical chromic acid final rinse solution (I) were tested. Three panels for each rinse plus three additional panels without a final rinse were prepared for evaluation of corrosion resistance per ASTM B117 salt spray testing. Results of salt spray testing are summarized in Table II. Two additional zinc phosphate/chromic acid final rinse panels were prepared for coating weight and crystal structure evaluation. The following chemical characteristics and processing parameters of the final rinses were used except where noted under the "Results" section of the individual trials.

TABLE II—Corrosion Resistance Summary
Rinse Solution
A
B
B2
C
D
E
F
H
Blank
I
Trial 1 failed
0.5 hr
1.5 hr
NP
0.5 hr
0.5 hr
0.5 hr
0.5 hr
2.0 hr
0.5 hr
NP
Trial 2 failed
1.0 hr
4.5 hr
NP
1.5 hr
1.5 hr
1.0 hr
1.5 hr
4.5 hr
1.0 hr
NP
Trial 3 failed
1.0 hr
2.5 hr
NP
1.0 hr
1.5 hr
1.0 hr
1.5 hr
4.5 hr
1.0 hr
NP
Trial 4 failed
1.0 hr
2.0 hr
3.0 hr
1.0 hr
NP
0.5 hr
NP
>72 hr
0.5 hr
0.5 hr
Trial 5 failed
NP
2.0 hr
2.0 hr
NP
NP
NP
NP
2.0 hr
0.5 hr
NP
ASTM D610 Rust Grade
2
6 or higher
6 or higher
1 to 2
0 to 1
0 to 1
1 to 2
6 or higher
0 to 1
0 to 1
NP=Not Performed

Solution "A" was a proprietary phosphoric acid/ammonium dimolybdate solution operated at 135F with a 30-sec immersion time. Solution concentration was 0.25%. Solution "B" and "B2" were proprietary alkaline sodium silicate solutions operated at 135F with a 60-sec immersion time. Solution concentration was 5.0% for solution B and 10% for solution B2. Solution "C" was a proprietary alkaline organic silane-containing solution operated at 110F with a 30-sec immersion time. Solution concentration was 1.5%. Solution "D" was a proprietary fluoride-containing powder operated at ambient temperature with a 60-sec immersion time. Solution concentration was 0.5 lb/100 gal. Solution "E" was a proprietary alkaline rust inhibiting solution without nitrites operated at ambient temperature with a 60-sec immersion time. Solution concentration was 2%. Solution "F" was a proprietary mildly acidic polymer solution operated at ambient temperature with a 120-sec immersion time. Solution concentration was 0.75%. Solution "H" was a typical chromate sealer bath containing 300 grams of chromic acid and 300 grams of phosphoric acid per 1,000 liters of bath solution operated at 160F with a 60-sec immersion time. Solution "I" was an experimental sealer bath containing 300 grams of molybdic acid and 300 grams of phosphoric acid in 1,000 liters of bath solution operated at 160F with a 60-sec immersion time.

Results

Trial 1 evaluated solutions A, B, C, D, E, F and H. The coating weight for this trial was 23.10 g/sq meter. The panels were examined using a scanning electron microscope (SEM). The crystal structure consisted of 35% large crystals approximately 30µm x 15µm and 65% smaller crystals approximately 15µm x 6µm. The crystal had a general block-like appearance.

Trial 2 evaluated solutions A, B, C, D, E, F and H. The coating weight for this trial was 16.10 g/sq meter. The panels were examined using SEM. The crystal structure consisted of 25% large crystals approximately 60µm x 15µm and 75% smaller crystals approximately 15µm x 6µm. The larger crystals retained the block-like structure seen in Trial 1, but the smaller crystals had a more plate-like appearance.

Corrosion Resistance Summary of Trial 1
Rinse
A
B
C
D
E
F
H
Blank
Hours to Failure
0.5
1.5
0.5
0.5
0.5
0.5
2.0
0.5

Trial 3 evaluated solutions A, B, C, D, E, F and H at half the initial concentrations described in the experimental procedure section. The coating weight for this trial was 14.8 g/sq meter. The panels were examined using SEM. The crystal structure consisted of 25% large crystals approximately 60µm x 15µm and 75% smaller crystals approximately 15µm x 6µm. The crystals appeared slightly more plate-like when compared to crystals from Trial 2.

Trial 4 evaluated solutions A, B, B2, C, E, H and I. Solutions A, C and E were twice the initial concentrations described in the experimental procedure section. The coating weight for this trial was 16 g/sq meter. The panels were examined using SEM. The crystal structure consisted of 15% large crystals approximately 80µm x 30µm and 85% smaller crystals approximately 6µm x 3µm. All crystals had a distinct plate-like appearance.

Corrosion Resistance Summary of Trial 2
Rinse

A
B
C
D
E
F
H
Blank
Hours to
Failure

1.0
4.5
1.5
1.5
1.0
1.5
4.5
1.0
ASTM D610
Rust Grade

2
6 or higher
1
0 to 1
2
2
6 or higher
0 to 1

Trial 5 evaluated solutions B, B2 and H. The coating weight for this trial was 16.6 g/sq meter. The panels were examined using SEM. The crystal structure consisted of 40% large crystals approximately 55µm x 25µm and 60% smaller crystals approximately 12µm x 3µm. All crystals had a distinct plate-like appearance.

The results of these experiments demonstrate the possibility of substituting an alkaline sodium silicate or a similar product for the chromate sealer specified in DOD-P-16232F or TT-C-490C. This conclusion is based on the comparable corrosion resistance of the alkaline sodium silicate with the chromate sealer in trials 2 and 5, and with the alkaline sodium silicate passing the 2-hour minimum salt fog test in trials 2, 3 and 4.

Corrosion Resistance Summary of Trial 3
Rinse

A
B
C
D
E
F
H
Blank
Hours to
Failure

1.0
2.5
1.0
1.5
1.0
1.5
4.5
1.0
ASTM D610
Rust Grade

2
6 or higher
1
0 to 1
0 to 1
1
6 or higher
0 to 1

There were differences in the phosphate coating structure and chemical composition between the various trials that introduced a variable to this investigation. This difference in chemical composition was noted by x-ray analysis on the SEM. Crystal structure and chemical composition variations in the phosphate coatings are the most likely explanations for the different failure rates among the various trials and demonstrate the importance of good, corrosion-resistant conversion coatings (maximum coverage without voids) prior to the final rinse, especially when using non-chromate rinses.

Corrosion Resistance Summary of Trial 4
Rinse

A
B
C
E
H
Blank
I
Hours to
Failure

1.0
2.0
3.0
0.5
>72
0.5
0.5
ASTM D610
Rust Grade

2
6 or higher
6 or higher
0 to 1
6 or higher
0 to 1
0 to 1

These variations could not be minimized because actual production conditions were used to apply the zinc phosphate coatings where salt spray and coating weight constitute the acceptance criteria. Only one variable, solution concentration, was evaluated in this study for the non-chromate rinses. The true capability of this product as a replacement for chromate sealers cannot be known without evaluation of other commercial zinc phosphate products, chemical compositions, work loading, temperature and immersion time to optimize this product's operating parameters. Various crystal structures and tank chemistries with each commercial zinc phosphate product may affect the optimization of these non-chromate final rinses.

Corrosion Resistance Summary of Trial 5
Rinse

B
B2
H
Blank
Hours to
Failure

2.0
2.0
2.0
0.5
ASTM D610
Rust Grade

7 or higher
6 or higher
7 or higher
0 to 1

Chromate rinses should continue to be used for phosphate processes until more research is conducted on sodium silicate rinses to maximize its corrosion resistance properties. Further investigation is needed to determine the optimum phosphate coating properties and tank parameters when using non-chromate rinses. The use of sodium silicate final rinses on coatings produced with other heavy zinc phosphate chemicals should be investigated as well as the use of sodium silicate final rinses on manganese phosphate coatings.



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