Laboratory Assessment of DoD Coating Adhesion 

on Ti-6Al-4V 

by Tom A. Considine, Chris E. Miller, and Brian E. Placzankis 

ARL-TR-3879 August 2006 

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Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-3879 August 2006 

Laboratory Assessment of DoD Coating Adhesion 

on Ti-6Al-4V 

Tom A. Considine, Chris E. Miller, and Brian E. Placzankis 

Weapons and Materials Research Directorate, ARL 

Approved for public release; distribution is unlimited.
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4. TITLE AND SUBTITLE 

Laboratory Assessment of DoD Coating Adhesion on Ti-6Al-4V 

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AH84 

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Tom A. Considine, Chris E. Miller, and Brian E. Placzankis 

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U.S. Army Research Laboratory 

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Aberdeen Proving Ground, MD 21005-5069 

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13. SUPPLEMENTARY NOTES 

14. ABSTRACT 

Titanium is used in industry to manufacture many consumer products such as softball bats and golf clubs. It is also widely used 

in aerospace and the defense industries. At issue is that titanium does not maintain a good adhesive bond with organic coating

systems. This study evaluates the adhesion of two nonchromate DoD primers, MIL-P-53022 and MIL-PRF-23377 Class N, 

relative to a direct to metal process using no primer. The evaluation method was based on ASTM D 4541 and used standard 

hydraulic adhesion testing equipment testing. The effectiveness of each method is then discussed with respect to strength values 

and failure modes. 

15. SUBJECT TERMS

coatings, titanium, adhesion, nonchromate, primer, pretreatment, abrasive blast, armor 

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Tom A. Considine 

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iii

Contents 

List of Figures iv

List of Tables iv

1. Introduction 1

2. Experimental Procedure 1

3. Results 2

3.1 MIL-P-53022 Primer.......................................................................................................2 

3.2 Grit Blast/No Primer .......................................................................................................2 

3.3 MIL-P-23377N Primer....................................................................................................4 

4. Discussion 5

5. Conclusions 8

6. References 9 

Distribution List 10 
iv

List of Figures 

Figure 1. Schematic of surface preparation and coating application steps.....................................1 

Figure 2. Pull-off hydraulic adhesion test (ASTM D 4541). ..........................................................2 

Figure 3. Post HATE-test MIL-P-53022 section of titanium block................................................3 

Figure 4. Data obtained from HATE test for MIL-P-53022 primered section. ..............................3 

Figure 5. Post HATE-test grit-blasted/unprimed MIL-DTL-64159 section...................................4 

Figure 6. Data obtained from HATE test for abrasive-blasted/unprimed section. .........................4 

Figure 7. Post HATE-test MIL-PRF-23377N section of titanium block........................................5 

Figure 8. Data obtained from HATE test for MIL-PRF-23377N primered section. ......................6 

Figure 9. Typical cohesive failure within MIL-PRF-23377N primer.............................................7 

Figure 10. Typical cohesive failure of the topcoat applied over MIL-P-53022 primer..................7 

List of Tables 

Table 1. Data obtained from HATE testing (values in psi). ...........................................................6 

Table 2. Failure mode location breakdown by percent...................................................................8 
1

1. Introduction 

With the necessity of titanium and its various alloys on the rise, so accordingly does the need to 

be able to coat them. Titanium is lighter than but just as strong as steel, twice as strong as 

aluminum, and extremely resistant to corrosion. It has many current applications, especially in 

the aerospace industry, and has many more proposed applications on the horizon. One small 

problem with titanium is that it does not maintain a good adhesive bond with organic coating 

systems; as such, coating delamination is fairly common. Past studies involving titanium on 

aircraft have brought this problem to light; therefore, the proposed goal of this report is to find a 

suitable non-chromate method to help enhance the adhesion of organic coatings on Ti-6Al-4V, or 

titanium grade 23 (NDCEE, 1997). 

2. Experimental Procedure 

An 8- × 7- × 1-in block of titanium (Ti-6A-4V) was pretreated via abrasive blasting, utilizing 

60-grit Al2O3 blast media, and then divided into three sections for different methods of coating. 

The top 3-1/4 in was primed according to spec with MIL-P-53022-10 (1992), the middle section 

received no primer and was masked, and the bottom 2-1/4 in was primed according to spec with 

MIL-PRF-23377N (2005) (see figure 1). All sections, including the remaining bare section in 

the middle, were then topcoated with MIL-DTL-64159 (2002) type II at a thickness of 

approximately 3 mil and allowed to set and cure. After curing, the panel was then cleaned of 

dust and subjected to the hydraulic adhesion testing equipment (HATE) testing in accordance 

with ASTM D 4541 (1989). A steel loading fixture (referred to as a dolly) is secured to the 

surface of the test area using an adhesive under laboratory conditions (see figure 2). In this case, 

each dolly was set using S-100 Instabond cycnaoacrylate and given a 24-hr period to cure. Only 

one dolly was adhered to the panel per pull to prevent false data during subsequent pulls due to 

shock from the initial pulling events. Data from each pull, both adhesive strength and failure 

mode, were recorded. Pulls that resulted in a failure of the adhesive were ignored. 

Figure 1. Schematic of surface preparation and coating application steps.

Grit Blasted Primed Top Coated 

MIL-P-50322 

MIL-PRF-23377N 

MIL-DTL-64159 
2

Figure 2. Pull-off hydraulic adhesion test (ASTM D 4541). 

3. Results 

3.1 MIL-P-53022 Primer 

As can clearly be seen from figure 3, the majority of pulls resulted in a cohesive failure of the 

topcoat. A few pulls resulted however in a slight cohesive failure of the primer around the center 

where contact was made between the tester and the surface. However, not a single pull-off 

resulted in any failures at the primer/substrate interface. Adhesive strength was fairly high, 

maintaining an average pull-off strength of 3545.65 psi (see figure 4). This relatively high 

average actually called for a slight change in the dolly size for the HATE test. While starting 

with 3/4-in-diameter dollies, the high strengths were beginning to exceed the capacity of the 

tester, and forced the use of a larger tester which used dollies of 7/8-in diameter. The switch 

allowed testing to continue and record maximum value of 4200 psi. 

3.2 Grit Blast/No Primer 

As can be seen in figure 5, there were a few instances of adhesive failure during the HATE tests. 

Most of the localization of the failure modes were cohesive within the top coat. The average 

adhesive strength on this section was 3056.25 psi (see figure 6), about 500 psi lower than the 

MIL-P-53022 primed section. Here, a maximum adhesion of 3590 psi and a minimum of 1800 

psi were achieved. 
3

Figure 3. Post HATE-test MIL-P-53022 section of titanium block. 

3546

295

3534 3510

121

4200

3060

0

500

1000

1500

2000

2500

3000

3500

4000

4500

Average STD DEV Geometric Mean Median 95% Confidence MAX MIN

Strength Values (psi)

Figure 4. Data obtained from HATE test for MIL-P-53022 primered section. 
4

Figure 5. Post HATE-test grit-blasted/unprimed MIL-DTL-64159 section. 

3056

490

3012

3180

240

3590

1800

0

500

1000

1500

2000

2500

3000

3500

4000

Average STD DEV Geometric Mean Median 95% Confidence MAX MIN

Figure 6. Data obtained from HATE test for abrasive-blasted/unprimed section. 

3.3 MIL-P-23377N Primer 

A cursory glance at this section reveals that cohesive failures were the majority here (figure 7). 

Most pulls resulted in a cohesive failure of the primer, while the remainder were cohesive 

failures in the top coat. Like the MIL-P-53022, not a single pull resulted in adhesive failure at 

the substrate. An average adhesive strength of 3498 psi was attained, with a maximum at 4200 

psi and a minimum of 3100 psi. 
5

Figure 7. Post HATE-test MIL-PRF-23377N section of titanium block. 

4. Discussion 

The intent of this study was to find a good, strong non-chromate primer that would properly 

adhere to titanium armor alloys. It has been found that titanium has an increased susceptibility to 

delamination; as such, a method for coating titanium armor that addresses and controls this issue 

was needed. It can be seen from figure 8 that both abrasive blasting followed by priming the 

titanium substrate with either MIL-P-53022 or MIL-PRF-23377N enhances the substrate’s 

ability to maintain coating adhesion. These findings coincide with a study done by Raytheon 

Electronics Systems and the Air to Air Joint Systems Program Office, where it was found that 

using MIL-P-53022 on a grit-blasted or equivalent substrate was the best method in coating 

titanium. Both primers maintained their adhesion to the substrate under higher strengths, on 

average, about 500 psi higher than the unprimed section (see table 1). It is interesting to note 

that each primer/coating performed almost identically when it came to the strength necessary for 

each pull-off failure. MIL-P-53022 maintained, on average, an advantage of about 50 psi over 

MIL-PRF-23377N. Although 50 psi is not significant in itself, the way in which each failure 

occurred needs to be investigated for clarification. 

The section of the plate, having only a topcoat applied direct to the abrasive-blasted substrate, 

was clearly the most susceptible to adhesive failure. In most every pull, at least a small amount 

of substrate was made visible. Pulls seemed to fail similarly based on their location on the plate. 

It is possible that these localized failure modes are a result of the uneven nature of abrasive 

blasting and would explain the results seen in figure 3. 

The section primed with MIL-PRF-23377N, as previously stated, performed admirably with 

respect to strength. However, the majority of the failure modes occurred within the primer layer 

(see figure 9). 
6

3498.42

262.51

3489.36 3520.00

118.04

4200.00

3100.00

0.00

500.00

1000.00

1500.00

2000.00

2500.00

3000.00

3500.00

4000.00

4500.00

Average STD DEV Geometric Mean Median 95% Confidence MAX MIN

Figure 8. Data obtained from HATE test for MIL-PRF-23377N primered section. 

Table 1. Data obtained from HATE testing (values in psi). 

 MIL-P-53022 DTM MIL-PRF-23377N 

Average 3545.65 3056.25 3498.42 

Std. dev. 295.11 490.45 262.51 

Geometric mean 3534.13 3012.04 3489.36 

Median 3510.00 3180.00 3520.00 

95% confidence 120.61 240.32 118.04 

Max. 4200.00 3590.00 4200.00 

Min. 3060.00 1800.00 3100.00 

It is plainly visible that the grit blast helped to maintain adhesive bond between primer and 

substrate; however, the primer itself failed cohesively under the stress. Very few of the pulls 

from the HATE test on this section coated by MIL-PRF-23377N resulted in failures of the top 

coat. Those that did have cohesive failures on the top coat also had places in the primer that 

failed cohesively as well. 
7

Figure 9. Typical cohesive failure within 

MIL-PRF-23377N primer. 

Finally, the section primed with MIL-P-53022 performed as well as the MIL-PRF-23377N, if not 

slightly better, with respect to adhesive strength. Each HATE pull within the MIL-P-53022 

failed cohesively within the top coat (see figure 10). From this, it can be said that the cohesive 

nature of MIL-P-53022 involves a stronger bond than that of the MIL-PRF-23377N (see table 2). 

Both primers, however, did bond well to the grit-blasted substrate, as neither section suffered one 

single adhesive failure. 

Figure 10. Typical cohesive failure of the 

topcoat applied over MIL-P53022 primer. 
8

Table 2. Failure mode location breakdown by percent. 

 MIL-P-53022 DTM MIL-PRF-23377N 

Cohesive – top coat 100 68.75 15.79 

Cohesive – primer 0 NA 84.21 

Adhesive – substrate 0 31.25 0 

5. Conclusions 

1. Pretreating titanium via abrasive blasting worked rather well overall. In the unprimed 

section, although many of the failures occurred at the substrate-coating interface, the 

adhesion strength was relatively high. 

2. The section primed with MIL-PRF-23377N held even better with higher pull-off values 

than DTM. The vast majority of the MIL-PRF-23377N included cohesive failures within 

the primer. 

3. The section primed with MIL-P-53022 outperformed the rest, sustaining no adhesive 

failures and very few cohesive failures within the primer. 

4. Since all pulls resulted in cohesive failures in the top coat, it can be said that abrasive 

blasting followed by priming with MIL-P-53022 is a viable method of coating titanium 

armor alloys. 
9

6. References 

ASTM D 4541. Standard Test Method for Pull-Off Strength of Coated Specimens Subjected to 

Corrosive Environments. Annu. Book ASTM Stand. 1989. 

MIL-P-53022-10. Primer, Epoxy Coating, Corrosion Inhibiting, Lead and Chromate Free 1992. 

MIL-PRF-23377N. Performance Specification, Primer Coatings: Epoxy, High-Solids 2005. 

MIL-DTL-64159. Coating, Water Dispersible Aliphatic Polyurethane, Chemical Agent 

Resistant 2002. 

National Defense Center for Environmental Excellence (NDCEE). Validation of Alternatives to 

Chromate-Containing Primer Coatings for Aircraft Exterior Mold Line Skins. Joint Test 

Protocol MD-P-1-1, December 1997. 
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