Chromium Free Corrosion Resistant Coatings

Mayaterials’ new silsesquioxane (SQs) based coating systems can be used to produce both hard, abrasion resistant conversion and primer coatings. SQ based spray coating systems provide highly adherent, very hard (5-6 H), and corrosion resistant coatings without chromium as demonstrated below.

Corrosion Resistance
Current corrosion resistant coatings employ chromate as a corrosion inhibitor. Chromate is both toxic and mutagenic but offers excellent corrosion resistance for aluminum alloy substrates. Therefore it is highly desirable to replace it with another corrosion inhibitor.

Mayaterials in work funded by the Air Force, has recently developed chromium free corrosion resistant coatings with our SQ coating systems. Our tests show that these SQ coatings offer exceptional properties.

In-house corrosion testing is conducted in an aerated 5 wt% saltwater bath which we find is more aggressive than the 2000 h salt spray tests often used for industry specifications such as ASTM B117. Spray coated substrates are scored per the ASTM B117 method and then immersed in the aerated bath for 200 h.

The following results offer a comparative analysis of our coatings against each other and commercially available alternatives. Two commercial coatings were tested in-house for comparison under our test conditions. One coating contains chromate, Alodine, and the other is a chromate-free conversion coating, Boegel. After 200 h in the corrosion bath the Boegel corroded severely and the Alodine passed as shown in Figures 1 and 2. Optical micrographs (OMs) of the region near a score of these coating systems are shown in Figure 2.

Figure 1. Results of 200 h corrosion tests on: Left) Commercial chromate-free Boegel conversion coating, Right) Alodine chromate containing coating.

Figure 2. OMs of scored portion of samples after 200 h corrosion test on: Left) Boegel conversion coating, Right) Alodine coating.

Four of our chromate-free SQ coating systems outperformed the commercial chromate-free conversion coating, Figures 3 and 4. Optical micrographs (OMs) of scores are shown in Figure 4.

Figure 3. Results of 200 h corrosion test on SQ coating systems: A) SQ1, B) SQ2, C) SQ3, D) SQ4

Figure 4. OMs of scores from 200 h corrosion test on SQ coating systems: A) SQ1, B) SQ2, C) SQ3, D) SQ4

Figures 5 and 6 show that two of our chromate-free SQ coating systems also had corrosion inhibiting properties similar to the chromated coating, Alodine. Micrographs of the region near a score of these coating systems are shown below in Figure 6.

Figure 5. Results of 200 h corrosion test on SQ coating systems: Left) SQ5, Right) SQ6

Figure 6. OMs of 200 h corrosion test on SQ coating systems: Left) SQ5, Right) SQ6

These latter two systems offer properties superior to the other Mayaterials systems but it is possible to also add epoxy and amine functionality to the surfaces of these coatings making it potentially possible to produce a conversion and primer coating in one step. Patents are pending.

Hardness
Pencil hardness tests are conducted according to ASTM D 3363 using an Elcometer 501 PHT. By optimizing the composition of these systems, we are able to achieve the highest measurable pencil hardness for all systems. The scale for measure pencil hardness is:

6H > 5H-H > F > HB > B-5B > 6B

Table 1. Pencil hardness for all SQ based conversion coatings

Coating Hardness SQ1 ≥ 6H SQ2 ≥ 6H SQ3 ≥ 6H SQ4 ≥ 6H SQ5 ≥ 6H SQ6 ≥ 6H

Adherence
To ensure proper adherence of the coating to a substrate’s surface, one-point bend tests are run according to ASTM D 552 Method B. Figure 7 provides a typical test result after a 200 h salt corrosion test. The coating is free from adhesion and cohesion failure meaning there is no cracking, crazing, or buckling of the coating on the bent surface.

Figure 7. Typical passing result from one point bend test on typical SQ coating

Epoxy Primer Coating Systems

Mayaterials’ silsesquioxanes offer considerable potential to develop highly adherent epoxy coatings with tailored properties. For example we can tailor CTEs, hardness and hydrophobicity. With our Janus cube systems we can also make multilayer materials. Their high thermal stability coupled with the ability to tailor properties at nanometer length scales makes them ideal for such applications including novel internal/external aircraft coatings, barrier coatings, no-flow flip-chip underfills, matrices for fiber reinforced composites, and many others.

Below are some of the epoxy systems we have developed. All current resin systems employ octaaminophenylsilsesquioxane (OAPS) as the curing agent. In the near future, we will greatly increase our catalog of coatings systems to include still lower viscosity epoxy systems, isocyanates and other related coating materials.

Formulation
Resins are formulated using OAPS (Ph-6), and OG (Q-4) and OC (Q-5) or one of the above epoxy compounds in a 1:1 epoxy to NH2 stoichiometry. Properties can be tailored by using one type of epoxy or mixtures of one or more, as suggested, but not limited to those shown below in Table 2.

Table 2. Coating Name Abbreviation Definitions

Coating Properties and Testing
After the coatings are completely cured (2-7 d), we test them to both Boeing spec and Milspec for such properties as hardness, adhesion, and chemical and corrosion resistance. The results from some of these tests are shown below in Tables 3-5.

Table 3. General Properties of OAPS Coatings

Property OAPS Coating Characteristic Color Transparent; slightly yellow Drying Time Tack free, 6 h at 19±2°C Curing Time 90% hardness, 4 d at 19±2°C Viscosity 16.5 s in Zahn #2 Cup

Table 4. Pencil hardness, dry, wet, and bend adhesion for several OAPS coatings

Hardness Dry Adhesion Wet Adhesion Bend Adhesion   ASTM Boeing ASTM Boeing   Spec* 1H-2H 4B 7 4B 7 4 OAPS-DGEBA 2-3H 4B 8 4B 8 1 OAPS-OG F-1H 4B 8 2B 4 4 OAPS-DGEBA+ 3-4H 5B 10 3B-4B 7 3 OAPS-OG+ 3-4H 4B 8 3B-4B 7 3 OAPS-Z 3-4H 5B 10 4B-5B 9 4 OAPS-TGMX 6H 5B 10 5B 10 4 OAPS-Os F 5B 10 5B 10 4

Table 5. Fluid resistances for some OAPS coatings

Hydraulic Fluid Lubricating Oil DiEGME     7 day 30 day Spec* 5 5 5 5 OAPS-Z 5 5 0 - OAPS-TGMX 5 5 5 5† OAPS-DGEBA 5 5 3 0

*Specs are according to both Boeing spec and Milspec
† This coating was cured at 100C for 2 h

Application
Coatings are sprayed onto 2024-T3 Al substrates using a M1-G HVLP spray gun. The coatings are then allowed to cure at room temperature.

Figure 8. M1-G HVLP spray gun used to apply our coatings to Al substrates.