Abstract... · Web viewAluminum alloys have unique characteristics that present different...

OPTIMIZING SURFACE PREPARATION OF ALUMINUM ALLOYS FOR COATING APPLICATION

Patrick Cassidy ([email protected]), Elzly Technology Corporation

Anthony Monda, Elzly Technology Corporation

Damien Ranero, Naval Surface Warfare Center Philadelphia Division

Anthony Eng, Naval Surface Warfare Center Philadelphia Division

Keywords: Aluminum, Surface Preparation, Abrasive Blasting, Pre-Treatments

ABSTRACT

The Navy currently performs surface preparation of aluminum alloys equivalently to that of steel alloys for the same coating systems. Based on lab observations, coating systems adhere differently and perform differently based on the substrate they are applied. Due to ongoing premature failures in the Fleet, the current methods for preparation of aluminum are being reevaluated through laboratory testing.

Aluminum alloys have unique characteristics that present different challenges from preparing and coating steel al-loys. The focus of this testing was on evaluating the mechanical surface preparation methods (abrasive blasting and hand tooling) used by the Navy to optimize the service life of coatings. The independent variables tested in -clude preparation method and surface morphology (i.e., effect of profile size and shape), contamination level (ef-fect of soluble salts), and overcoat window prior to priming (effect of aluminum oxidation layer). The impact of these variables on the long-term performance of the coating systems was measured.

The information compiled and presented in this paper will aid the draft of a new industry surface preparation stan -dard for aluminum substrates that will be available for use by the Navy, Army, Marine Corps, Coast Guard, other DoD services and industry.

INTRODUCTION

The use of structural aluminum alloys is becoming more prevalent in the Fleet. The alloys are typically coated to provide corrosion protection and for aesthetic/camouflage reasons. The requirements for preparation and applica-tion of these coatings are outlined in Navy specifications such as NAVSEA Standard Item (NSI) 009-32 and Naval Ship’s Technical Manual (NSTM) Chapter 631. However, the life cycle of these coating systems applied over alu-minum is not meeting the needs of the Fleet. A previous paper presented at the DoD Allied Nations Technical Corrosion Conference in 2017 [1] identified improper surface preparation as the potential primary cause of failure of coatings over aluminum substrates across the Fleet. Feedback from the waterfront, though anecdotal, tended to support this supposition. Additionally, previous research performed by the Navy showed improved coating per-formance when comparing mineral abrasive blasting to glass bead blasting [2], and abrasive blasting to power tool preparation [3, 4].

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2019 Department of De-fense – Allied Nations

Technical Corrosion Con-Paper No. 2019-XXXXXX

The primary variables associated with surface preparation identified by previous work include preparation type, profile morphology and depth, chloride contamination, and degree of visual cleanliness/oxide layer formation. These are the main variables that form the backbone for the current ongoing laboratory testing.

The current ongoing laboratory testing is being performed in a two-part program, conducted in parallel. The first part is exploring the effect of surface preparation method and profile morphology on nonskid and topside system coating performance over two common Navy aluminum alloys. The second part is exploring the effect of visual cleanliness/oxide layer formation and chloride contamination on nonskid and topside system coating performance over two common Navy aluminum alloys. The testing consists of one-year minimum atmospheric exposure in a natural environment with daily salt spray. Ongoing visual inspections are being conducted every three months. Af-ter testing the test samples will be inspected for visual breakdown as well as evaluated destructively for material loss of the aluminum substrate.

Fleet In-service Coating Failures

Premature coating failures are being observed across the Fleet over aluminum substrates. The failure locations appear primarily in hard to coat geometric areas topside, like stiffener structures, and on decks.

Nonskid failures on aluminum catwalks have twice been reported recently. Both failures occurred in the same alu -minum catwalk areas on the same ship; the second failure occurred after only 2.5 months in service. The first re -port from the waterfront representative measured profile ranging from 4.9-10.9 mils. Some areas shown in the photos have obviously been over-prepared using power tools and grinders, yet others reveal a lack of profile and the waterfront representative comments that the “catwalk deck plates appear to have never been abrasive blasted.” Figure 1 shows photos of the failures and underlying aluminum. The upper left photo shows the overall condition while the upper right photo shows lack of underlying profile. The bottom left photo shows unsuitable pro-file made using a deck grinder, and the bottom right photo shows surface preparation using a rotary sanding disc.

The waterfront representative believes that the continuing failures of nonskid on the catwalks are due to inade-quate surface preparation. In some areas there has been no prior surface preparation, and in other areas Ship’s Force has prepared the deck with power tools, but has imparted very deep, non-angular profiles, unsuitable for coating application. Anecdotal evidence from other ships states that during new construction the catwalks only had one mil profile prior to coating application.

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Technical Corrosion Con-

Figure 1Nonskid Failures of Aluminum Catwalks

Figure 2 shows some pictures of the failures on the same catwalks in December 2016, only 2.5 months after the previous failures were fixed. Ship’s Force observed the aluminum exposed to be “pretty smooth to the touch which may reveal a lack of profile for adequate adhesion.”

Figure 2Nonskid Failures of Repaired Aluminum Catwalks

Historic inspections performed by NSWC showed similar delamination of nonskid on aluminum decks aboard sev-eral other ships. Examples are shown in Figure 3. Lack of adequate surface profile was cited as a factor in each of these cases.

Further inspections performed as a part of this test program revealed exterior ship structure was exhibiting mas-sive coating delamination (50-60%) coating failure on the underside structural members, Figure 4. The coating was easily removed in pieces approaching a square foot, exhibiting extensive catastrophic delamination. Limited areas of delamination were also seen on the nonskid deck surface.

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Figure 3Nonskid Failures on Aluminum Decks aboard Various Ships

Figure 4Coating Failures on Aluminum Structure

Literature Review

In a previous study conducted by NSWC in 2017 on the sensitization of aluminum they found that for the most op -timal coating adhesion and performance on sensitized aluminum, abrasive grit blasting to Society of Protective Coatings (SSPC) SSPC-SP 10 is recommended over power tool surface preparation (i.e., SSPC-SP 11). [3]

Another study performed by NSWC in 2011 [2] further identifies optimum media for abrasive blast preparation of aluminum substrates for coating performance. The program tested cathodic disbondment of epoxy coatings over

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aluminum panels prepared using various abrasive blasting media types, specifically fine and coarse glass bead, and fine and coarse aluminum oxide. The study found that the panels with the least coating disbondment were those that were prepared with aluminum oxide grit. The coarse aluminum oxide grit resulted in the best coating system performance. The authors concluded that this was due to the angularity of the profile imparted by the alu-minum oxide, vice the rounded profile obtained using glass bead.

Obviously, there is a balance when selecting a grit type and size. The larger the size the more angular the profile and the deeper the depth of profile. However, too large mesh size media will result in over-preparation and re-moval of too much material when abrasive blasting aluminum.

The standard most often cited by the Navy for steel is SSPC-SP 10. This also seems to be used often for alu-minum even though the scope of the standard states that it is for steel only. The associated visual standard, SSPC-VIS 1, has no equivalent aluminum reference standard.

Aluminum substrates are more difficult to inspect than their steel counterparts. Aluminum does not exhibit easily observable signs of oxidation like the rust back or flash rust which occurs on steel. Since SSPC visual standards are not available for determining cleanliness levels of aluminum prepared surfaces, instituting more stringent cleaning requirements may be more effective than visual standards for aluminum. Soluble salt contamination checks may need to be instituted in all cases of preparing aluminum.

Resources discussing the formation of the aluminum protective oxide layer indicate that this layer is a poor bond-ing layer for coating systems when it naturally occurs over long times. [4] An NSWC study in 2013 revealed if a power tool surface preparation method is used that a reduction of coating performance and adhesion can be ex-pected the longer the bare aluminum substrate is left exposed. They recommend that the coating application be performed immediately after surface preparation. [5]

GAP ANALYSIS

Based on the literature review [6] a gap analysis was conducted to determine the need for future testing and what test data is needed by the Navy to improve the way they prepare aluminum for coating application. The following areas were identified as having insufficient data:

Optimum standard grit type and size, profile depth, angularity and blast parameters (pressure, standoff, etc.) for abrasive blast preparation of aluminum

Effect of aluminum alloy type on abrasive blasting requirements and coating performance Effect of soluble salt contamination on aluminum alloy preparation Effect of the time and environmental conditions of aluminum exposure between preparation and priming

TEST PROCEDURE

Eighty-eight 6” x 12” x 0.25” aluminum panels were procured, half were 5XXX series alloy and half were 6XXX se-ries alloy. Equal number sets of panels were prepared using the preparation methods outlined in Table 1.

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Table 1Testing Prep Methods

System Preparation

Method Description

1A Rotary Disc Sander

Hand tool preparation to cleanliness equivalent to SSPC-SP 11, 100 grit paper, <1 mil profile, burnished

1B Needle Gun Hand tool preparation to cleanliness equivalent to SSPC-SP 11, >2 mil profile

1C Abrasive Blast 1

Abrasive blasting to cleanliness equivalent to SSPC-SP 10, 80-100 mesh aluminum oxide abrasive, 1-2 mil target angular profile

1D Abrasive Blast 2

Abrasive blasting to cleanliness equivalent to SSPC-SP 10, 30-40 mesh aluminum oxide abrasive, 3-5 mil target angular profile

1E Shot Blast 1

Abrasive blasting to cleanliness equivalent to SSPC-SP 10, stainless steel shot mesh size 40-70 (S-30/SAE S-110), 5-8 mil target rounded profile

1F Shot Blast 2

Abrasive blasting to cleanliness equivalent to SSPC-SP 10, stainless steel shot mesh size 18-25 (S-60/SAE S-330), 8-10 mil target rounded profile

In part one testing, duplicate panels of each preparation technique (forty panels total, half of each alloy) were coated with two different coating systems. Coating system one was a qualified nonskid system in accordance with the requirements of NSI 009-32. Coating system two was a qualified topside coating system in accordance with the requirements of NSI 009-32. Coating system two was further broken down into sets of panels with direct-to-metal polysiloxane and the traditional epoxy-polysiloxane system. Prior to painting, the contamination levels of all panels were verified using conductivity with the Bresle patch method. Part one panels were below 30 µS/cm be-fore painting and were used as a control group for part two panels. Profile morphology on each panel was charac -terized using a digital profilometer in accordance with ASTM D 4417 Method B and a stylus profilometer in accor -dance with ASTM D 7127.

In part two testing, forty-eight panels (half of each alloy) were contaminated at three levels. One-half of each panel set was further conditioned between contaminating and painting. Panels were contaminated at three differ -ent contamination levels using a micro-pipet to produce a nominal 0.5” diameter contaminated area with solutions of varying salt concentration, Figure 5. Panels were placed in a low humidity environment to encourage rapid evaporation of the contamination solution. The doping levels were as follows: Doping Level 1 – Salt Water; Dop-ing Level 2 – ½ Salt Water ½ Tap Water Mix; Doping Level 3 – Tap Water.

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Figure 5Part Two Panel Doping Location and Layout

A separate witness panel was used to measure conductivity levels of each doping level as applied to the panel using the Bresle Patch method. All doping was done with ASTM D 1141 synthetic seawater. The twenty-four pan-els for pre-exposure before painting were subjected to a high humidity environment to promote oxide layer growth and simulate extended exposure periods prior to painting. Panels will be pre-exposed for two different time peri -ods, one week and two weeks, twelve panels at each time period. This provided three overcoat periods: overcoat-ing within 24 hours with no pre-exposure, overcoating after pre-exposure for one week, overcoating after pre-ex-posure for two weeks.

After doping and pre-exposure duplicate part two panels were coated with the same two coating systems as part one panels. The panels were scribed using a non-ferrous 1/8” diameter mill bit. All panels from part one and part two of testing were placed in atmospheric exposure in Vineland, New Jersey. The panels are sprayed daily with artificial sea water (ASTM D 1141). The topside panels are exposed in a traditional 45-degree orientation. The nonskid panels are exposed in a horizontal (flat) orientation to simulate a ship deck.

INTERIM RESULTS

While the overall exposure is expected to last twelve months or more, interim inspections have been conducted on all panels every three months. Panels are rated visually for coating breakdown and blistering in accordance with ASTM D 610 and ASTM D 714 respectively, and scribe cutback is estimated visually at the maximum point. Scribe cutback at each interim inspection will be estimated visually without destructive evaluation.

The three-month inspection point showed initiation of failure on disc sander and needle gun prepared panels non-skid panels. The failure was delamination of the primer from the substrate around edges of the panel and at the

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scribe, Figure 6. The panels also showed cracking of the nonskid topcoat, Figure 7. No abrasive blasting prepared panels showed evidence of failure.

Figure 6Three-Month Nonskid Delamination

Figure 7Three-Month Nonskid Cracking

There was no apparent performance difference between alloys and the three doping/pre-exposure conditioning periods. Soluble salt contamination and overcoat window didn’t seem to effect coating performance as much as the type of surface preparation on the nonskid panels.

The three-month inspection also showed initiation of failure along the scribe of non-doped topside needle gun pre-pared panels. The failure manifested as lifting of the coating system from the substrate, Figure 8. Disc sanded and aluminum oxide abrasive blasted prepared non-doped panels showed no evidence of failure at three months.

On the salt-doped topside panels the disc sanded panels showed initiation of failure between the primer and sub-strate at doping locations along the scribe.

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Figure 8Three-Month Disbondment of Topside Coating

Topside panels prepared with 30-40 mesh aluminum oxide showed initiation of failure between the primer and substrate at doping locations along the scribe for control, non-pre-conditioned panels. Topside high-humidity pre-conditioned panels showed no evidence of failure, contrary to what we expected to see.

The six-month inspection point on the nonskid panels showed failures on all power tooled panels, regardless of doping level or pre-conditioning. Most panels showed delamination from three or all four sides as well as at the scribe, Figure 9. No failures were observed on any of the nonskid abrasively blasted panels. Again, soluble salt contamination and overcoat window didn’t seem to have as much effect on performance as the surface prepara-tion method.

Figure 9Six-Month Power Tool Nonskid Failures

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At the six-month inspection on the topside test panels the power tool prepared panels showed extensive failures regardless of salt contamination and overcoat period. Non-doped and non-pre-conditioned needle gun prepared panels showed major disbondment, Figure 10. Non-doped disc sanded panels showed initiation of failure along the scribe. Doped disc sanded panels showed delamination in the scribe only on 5XXX series alloy panels but major delamination on 6XXX series alloy panels at all three pre-conditioning levels, Figure 11. Interestingly, no test panels with direct-to-metal polysiloxane over power tool prepared substrate showed evidence of failure. All abrasive blasted panels, regardless of aluminum oxide grit size, salt contamination level and pre-conditioning pe -riod, showed no indication of failure.

Figure 10Six-Month Topside Needle Gun Failure

Figure 11Six-Month Topside Disc Sand Failure

At the end of the exposure period, final visual inspections for rust-through/coating breakdown and blistering will be taken on all panels and panels will be destructively evaluated. The coating will be removed around the scribe, and a final maximum scribe measurement will be taken. All panels will be probed for coating failure using the adhesion knife test, ASTM D 6677. Each panel will have the coating system and all corrosion products removed using a chemical paint stripper followed by glass bead blasting. Material loss will be evaluated on the panel face. Pit depth measurements will be taken in the holiday area (scribe and scribe undercutting area) and on the panel face, as applicable.

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CONCLUSIONS

There is a knowledge gap in the Navy as it relates to mechanically preparing aluminum substrates properly. Previ -ous NSWC testing showed the effect of surface preparation methods on coating performance. The literature re-view portion of this program identified paths for future testing in support of specification standardization and the development of an industry surface preparation standard. A test plan was developed to test the effect of type of preparation method, effect of salt contamination, and effect of overcoat window/oxide layer formation on coating system performance over aluminum alloys.

Three and six-month inspections on exposed test panels showed that abrasive blasted panels prepared with ei-ther grit size exhibited no signs of failure on topside or nonskid panels, regardless of salt contamination level or overcoat window.

Power tool prepared panels showed rampant failures on both nonskid and topside coating systems very early in the exposure period. The effects of overcoat window and salt contamination are just beginning to affect the extent of failure for power tool prepared topside coating systems.

Through the six-month inspection, the effects of salt contamination and overcoat window are not fully evident, the exposure will continue for at least an additional six months. It is evident so far, though, that the type of surface preparation performed (i.e., abrasive blasting versus power tooling) has a large impact on coating performance. This confirms the suspicions behind the failures seen in the Fleet.

ACKNOWLEDGMENTS

The authors would like to thank the Naval Sea Systems Command (NAVSEA) 05PF Painting Center of Excel-lence (PCOE) program for funding the work reported herein.

RESOURCES

1. P. Cassidy, D. Ranero, and B. Hicks. Optimized Surface Preparation of Aluminum Substrates for Coating Ap-plication. 2017 Department of Defense Allied Nations Technical Corrosion Conference. Birmingham, AL. Paper No. 2017-874415, August 2017.

2. J. Delbridge and R. Hays. Surface Preparation Methods for Aluminum Substrates. NSWCCD Letter Report 61-11-086, March 2011.

3. R. Park, K. Chasse, and D. Ranero. Corrosion and Coating Systems Evaluation for 5xxx Series Marine Grade Aluminum. NSWCCD Letter Report 61-TR-201X/XX February 2016, DRAFT.

4. C. Munger. Corrosion Prevention by Protective Coatings. NACE International, 1999.

5. D. Ranero. Optimization of Power Tool Surface Preparation Requirements for Nonskid System Application to Aluminum Substrates. NSWCCD Letter Report 61-13-062, May 2013.

6. D. Ranero, B. Hicks, A. Eng, P. Cassidy. Optimizing Surface Preparation of Aluminum Substrates. NSWCPD Letter Report Ser. 33/342. August 2018.

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