Moisture Testing Coating Performance on Metal Materials

Nolan Wells, P.E., Ralph E. Moon, Ph.D., Nizar Alrafie, EI GHD, Building Sciences Dept., 5904 Hampton Oaks Parkway Suite F, Tampa, FL

33610; and Brett Davis Pacific Architects and Engineers, MacDill AFB, 7621 Hillsborough Loop Dr.

MacDill AFB, 33621. Contact: PH (813) 971-3882; FAX (813) 971-1862, email: Nolan.Wells@ghd.com

Abstract

This study represents a survey that examined the performance of various coatings (i.e., White 2-Part Epoxy, Clear 2-Part Urethane, White Acrylic Roof Coating, White Silicon Roof Coating, Rust Preventer Primer, Rust Converting Primer, Behr® Styrene Acrylic Primer and Sealer, Rust-Oleum® Oil Based Primer And Rust-Oleum® Latex Paint). The coatings were applied to metal surfaces (i.e., aluminum, plain steel, galvanized steel, zinc-plated steel and rusted steel) exposed to fresh water, elevated relative humidity (RH), salt water and ultraviolet radiation (UV) to determine the corrosion failure rates. The study revealed the following results: 1) silicon displayed the best overall performance for inhibiting rust, 2) for pre-rusted metal surfaces White Silicon Roof Coating provided the best corrosion prevention followed by the Rust Converting Primer and the 2-Part Epoxy. 3) The worst performance was obtained following the application of White Acrylic Roof Coating, Rust-Oleum® Latex Paint and Rust Preventing Primer on plain steel and zinc-plated steel. 4) The salt water environment displayed the most aggressive corrosion. 5) 60 days of exposure to UV light caused some coatings to yellow or become brittle. 6) For the primers, Behr® Styrene Acrylic was best on aluminum and galvanized steel, while the Rust-Oleum® Oil Based Primer performed best on plain steel and zinc-plated steel. The overall performance of the remaining coatings in diminishing order were as follows: Rust Converting Primer, White Acrylic Roof Coating, White 2-Part Epoxy, 2-Part Urethane, Rust Preventing Primer and Rust-Oleum® Latex Paint.

Introduction

The occurrence of corrosion experienced by military assets such as aircraft, naval vessels, test facilities, administrative buildings, hangers or storage facilities threatens “mission readiness” and poses recurring maintenance costs. We recognized that since many military facilities are located near aggressive environments (high humidity, high annual rainfall, desert conditions, salt water environments) testing various coatings on a variety of common metal surfaces may identify those most suited for application.

The selection and application of effective coatings are an essential corrosion management tool; however, they are also an essential component in the management of facility maintenance and effective use of financial resources. This study emphasizes the importance of testing to determine the most appropriate coating type and application thickness. The test results demonstrated several unexpected features of coating behavior and identified unique differences of which we were unaware.

Materials and Methods

The study began by implementing effective surface preparation techniques for each type of metal surface. Surface preparation techniques employed several methods to remove residues such as cutting oils, dust, debris and finger prints. Several cleaning solvents were considered: 1) acetone, 2) mineral spirits, 3) surfactants (Dawn dish soap) and 4) degreaser. Among these cleaning solvents, a combination of dish soap and water rinse (no soaking) followed by hand wiping with mineral spirits (no residual deposits) on a cotton cloth was employed. Exemplars were allowed to dry 24 hours before coating applications.

The coatings were applied to the metal exemplars using angled paintbrushes (Wooster. 1” and 2” width) for enhanced precision when applied onto 3” x 4” exemplars. The test procedure examined five metal surfaces, nine coating products (including four primers) and two thicknesses for a total of 305 exemplars representing 80 combinations of exposure in each of the five environments (including controls and duplicates).

Five metal surfaces (i.e., aluminum, plain steel, galvanized steel, zinc-plated steel and rusted steel) were tested following the application of seven coatings (i.e., white 2-part epoxy, clear 2-part urethane, white acrylic roof coating, white silicon roof coating, rust preventer primer, rust converting primer, Behr® styrene acrylic primer and sealer). The six exposure conditions (control, elevated RH, fresh water, salt water, UV light and UV light with fresh water). Six of the nine coatings (2-part Epoxy, 2-Part Urethane, White Acrylic Roof Coating, White Silicon Roof Coating, Rust Preventing Primer, Rust Converting Primer) were obtained from APOC A Division of Gardner Industries, 4161 East 7th Ave Tampa, Florida 33605. Among the remaining three coatings the Oil Based Primer and Latex Paint were manufactured by Rust-Oleum® and the Styrene Acrylic Primer was manufactured by Behr®.

One or two applications of each coating was applied to evaluate the influence of thickness on corrosion performance. The first coat thickness range of each applied

coating varied with White Acrylic Roof Coating the thinnest (1.8 mils) and White Silicon Roof Coating the thickest (13.7 mils). The second coat thickness range varied from 3.8 mils (Rust Preventing Primer) to 31.8 mils (White Silicon Roof Coating). A summary of coating thickness is represented in Table 1. One-half of the opposite side of each exemplar was coated with the remaining two primers in one coat (Rust-Oleum® Oil Based Primer and Behr® Styrene Acrylic Primer).

Table 1. Thickness (mils) of coatings applied to surface in one or two coats

The acrylic and polycarbonate test apparatus was constructed with four isolated and independently controlled chambers. Within each chamber, independent vertical shelves were constructed to support the metal specimens while being exposed to fresh water, salt water, elevated RH, and a control environment at ambient conditions (76°F and 50% RH). Vertical shelves were constructed that could accommodate 35 stacked test exemplars in each chamber (Photo 1).

The UV test chamber (4ft length x 2ft width x 8.5 inches height) was constructed of ¼ inch plywood equipped with four UV flood lights (20 w) at a wavelength between 315-400 nanometers (UVA) positioned in the chamber lid approximately 8 inches above the exemplars. The exemplars were placed horizontally, in a side by side configuration with the UV lights positioned directly above. This configuration allowed similar exposure conditions for each specimen (Photo 2).

Five test chambers (salt water, fresh water, elevated RH, control and UV) were used with two exemplars of galvanized steel, plain steel and aluminum in each. One exemplar of rusted steel and zinc-plate steel were also inserted into the five test chambers. A vertical, horizontal-mounted polycarbonate storage system with individual shelves was used to support each exemplar exposed to salt, fresh, control and elevated RH. The exemplars were placed horizontally in a stacked sequence to accommodate the 60+ test samples. Among the vertical shelf systems exposed to salt and fresh water, the shelves were angled 10o off center to encourage drainage and minimize contact with water as it drained from the exemplars above (Photo 3).

1-Coat2-Coat

Measurements displayed in mils (1 mil = 0.001")7.5

3.1 7.8 4.18.3 9.3 8.9 31.8 3.8 8.3

Behr® Styrene Acrylic Primer

Rust-Oleum® Oil Based

Primer

Rust-Oleum® Latex Paint

4.3 6.5 1.8 13.7 2.6 5.4

White 2-Part Epoxy

Clear 2-Part Urethane

White Acrylic Roof Coating

White Silicon Roof Coating

Rust Preventer

Primer

Rust Converting

Primer

Photo 1: Test chamber with vertical shelves for exemplars

Photo 2: UV test chamber with exemplars laid side-by-side

Photo 3: Salt water test chamber with angled vertical shelves, fresh water (left) and salt water (right)

Results

Coating performance was evaluated using qualitative criteria: visible presence of corrosion, surface texture (smooth, small bumps, bubbling, raised spots and large bubbling) and delamination. Coatings were ranked according to a number of samples that displayed an approximate percent area of visible corrosion (0-4.9%, 5-24.9%, 25-74.9% and >75%) on the surface independent of apparent changes to the surface texture.

Best and Worst Coatings

The five metals and seven different coatings displayed different characteristics when exposed to salt water, fresh water, elevated RH, and UV radiation. Based on a comparison among all the coatings, White Silicon Roof Coating displayed the best overall performance for inhibiting rust and maintaining a smooth, unblemished surface after application (Table 2). The overall performance of the remaining coatings in diminishing order were as follows: Rust Converting Primer, White Acrylic Roof Coating, White 2-Part Epoxy, 2-Part Urethane, Rust Preventing Primer and Rust-Oleum® Latex Paint.

Table 2. Summary of coating performance identifying the best corrosion performance in green. Performance was assessed based on (1) visible corrosion and (2) surface conditions (see legend).

Among the five metals tested, the worst performance was obtained following the application of White Acrylic Roof Coating, Rust-Oleum® Latex Paint and Rust Preventing Primer on plain steel and zinc-plated steel (Photos 4-7).

Photo 4: Zinc-plated steel with acrylic coating in salt water after 77 days (left), uncoated control (right)

Photo 5: Zinc-plated steel with latex coating in salt water after 77 days (left), uncoated control (right)

Photo 6: Plain steel with rust preventing coating in salt water after 77 days (left). Uncoated control (right)

Photo 7: Plain steel with latex coating in salt water after 77 days (left). Uncoated control (right)

Primer Performance

The corrosion resistance exhibited by Behr® Styrene Acrylic was best on aluminum and galvanized steel, while the Rust-Oleum® Oil Based Primer performed best on plain steel and zinc-plated steel (Table 3).

Table 3. Summary of primer performance using styrene acrylic and oil-based primers. Rankings were based on (1) percentage visible corrosion and (2) surface conditions.

Metal WaterSaltFreshSaltFreshSaltFreshSaltFreshSaltFresh

1 A 0-4.9%2 B 5-24.9%3 C 25-74.9%4 D >75%56

Raised SpotLarge BubblingDelamination

Corrosion Percentage

Best performance

SmoothSmall BumpsSmall Bubbling

B3C2 C2C2

Rusted Steel

C5C1 A3

Zinc-Plated Steel

B2

Surface Conditions

A3A1 A3A1

Galvanized Steel

A3C1 B5C1

Plain Steel

A1A1 A3

Behr® Styrene Acrylic Primer

Rust-Oleum® oil based primer

AluminumA1

One or Two Coats?

Silicon performed the best whether applied with one or two coats on any metal surface. The Rust Converting Primer performed best with two coats. The 2-Part Epoxy and Urethane performed better with two coats. The Rust Preventing Primer performed the same whether one or two coats were applied. The worst performance was obtained from the white acrylic coating, however, two coats performed better than one. In most applications, two coats performed better than one to prevent visible corrosion (Table 4).

Table 4. Comparison of one and two coats in fresh and salt water. Coatings that exhibited the largest difference in corrosion are identified in orange. All other applications showed similar performance whether one or two coats were applied.

Metal Performance

Under both conditions of fresh water and salt water exposure galvanized steel and aluminum performed the best with all coatings.

Among the rusted steel exemplar Silicon performed best followed by the Rust Converting Primer and the 2-Part Epoxy.

The zinc-plated steel and plain steel exemplars showed the poorest performance with White Acrylic Roof Coating, Rust Preventing Primer and the Rust-Oleum Latex Paint by showing advancement in the visible amount of corrosion (Table 2).

Best Choices among Metals and Coatings

Aluminum surfaces would be best protected by White Acrylic Roof Coating and Silicon Roof Coating. Rust Preventing and Rust Converting Primer would also provide measurable corrosion prevention.

Metal WaterSaltFreshSaltFreshSaltFreshSaltFreshSaltFresh

ABCD

Rust Converting

Primer

White 2-Part Epoxy

Clear 2-Part Urethane

White Acrylic Roof Coating

White Silicon Roof Coating

Rust Preventer

PrimerAA

AA AA AA AA AA AAAA BA AA AA AA

AAAA BB BA AA CC BABB BB DD AA DD

AAAA AA AA AA AA AAAA AA AA AA AA

AAAA BB DA AA DD AABA BC DD AA DD

BBAA CC BC AA BB AABB BB CC AA BBRusted

Steel

Zinc-Plated Steel

Galvanized Steel

Plain Steel

Aluminum

DDBBCCCB

Rust-Oleum® latex paint

AAAADDDDAAAA

6-25%26-74%>75%

Corrosion Percentage0-5%

One Coat

Two Coat

Labeling Legend

Performance Change

AA

White Silicon Roof Coating and the Rust Converting Primer best protected the zinc-plated steel and plain steel. 2-Part Epoxy could also provide improved corrosion prevention.

Galvanized steel exhibited the best protection with the White Silicon Roof Coating, Rust Converting Primer and the 2-Part Urethane.

For pre-rusted metal surfaces, White Silicon Roof Coating provided the best corrosion prevention followed by the Rust Converting Primer and the 2-Part Epoxy (Table 2).

Fresh and Salt Water Exposure

Salt water induced more visible corrosion than fresh water exposure. The plain steel and zinc-plated steel displayed the most visible corrosion when compared to aluminum, galvanized and rusted steel.

Controls

Two types of controls were incorporated into the test methodology. Among the five test metal exemplars all exhibited corrosion or surface oxidation following exposure to fresh and salt water over a 77 day period. Zinc-plated steel, plain steel and aluminum all exhibited the most severe corrosion (Photos 8-10).

A set of control samples under ambient conditions were prepared that were identical to those exposed to the fresh, salt and elevated RH test chambers. Among the different types of coatings tested the exemplars with Rust-Oleum® Latex Paint and White Acrylic Roof Coating displayed minor surface corrosion (Photos 11 and 12). All other exemplars exhibited no visible corrosion on either the coatings or the primers.

Photo 8: Plain steel control in salt water after 77 days (left), ambient control (right)

Photo 9: Zinc-plated steel control in salt water after 77 days (left), ambient control (right)

Photo 10: Aluminum control in salt water after 77 days (left), ambient control (right)

Photo 11: Plain Steel Acrylic control ambient after 77 days spots of surface corrosion (right), UV only (left)

Photo 12: Plain Steel Latex control ambient after 77 days spots of surface corrosion (right), UV only (left)

Conclusions Several conditions contributed to coating failure and metal corrosion. Coating failure is commonly associated with moisture exposure. We found that moisture alone expressed corrosion over time with certain types of coatings with UV light initiating little to no observable change in corrosion during the 60-day exposure period.

The testing of various coatings and primers to fresh and salt water exposure revealed their vulnerabilities and there most suited applications. Galvanized metal and aluminum are best suited for corrosive environments. The non-aqueous based coatings performed better than the water-based coatings.

Among the coatings tested the 2-part mixes (epoxy and urethane) were the most difficult to prepare for application, clean and avoid inhalation risks. Best suited for application in an open-air environment.

Silicon exhibited the highest viscosity and thickest application.

Surface preparation techniques for 2-Part Epoxy was difficult for all metal surfaces due to its sensitivity to any minor surface residues.

All coatings were applied with a brush. Some coatings (Rust-Oleum Latex Primer and White Acrylic Roof Coating) exhibited corrosion where the bristles applied the thinnest material. In these applications, two coatings were preferable.

Coatings failure analysis must consider a number of contributing factors. Installation techniques, temperature changes, exposure to UV light, thermal expansion, and water erosion can all contribute to the failure of coatings.

Oil based and latex exhibited severe corrosion when exposed to fresh water.

Further research is necessary to identify how the substrate, primer and finish coat interact in response to temperature change and moisture exposure. The authors recognize that damage from UV exposure is a prolonged occurrence that will manifest over months to years of repeated exposure. This study was not conducted for sufficient duration to evaluate this aspect of failure. Error

Inherent variability of test materials and procedures were recognized and minimized during the coatings test. Influences such as temperature and minerals in the water contributed minor variations in the collected data. The two part mixtures required exact ratios to achieve the proper performance.

Surface preparation of the metal exemplars was conducted using a surfactant (Dawn® dish detergent) and low residue mineral spirits. Efforts were made by the authors to remove as much surface residues as possible.

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