Due to the severity of their operating environments, turbine blades often require environmentally protective coatings on the surfaces of their airfoils and platforms exposed to the hot gas path. Diffusion coatings such as chromide, aluminide, and platinum aluminide coatings are widely used as environmental coatings in gas turbine engine applications because of their oxidation resistance. Such coatings, which are typically applied to the internal and external surfaces of a blade, are produced by a thermal/chemical reaction process that results in the near-surface region of the substrate being enriched with, depending on the type of coating, chromium, aluminum, platinum, etc., as well as intermetallics that form as a result of reactions between the deposited corrosion-resistant specie(s) and the substrate material. Diffusion coating processes typically take place in a reduced and/or inert atmosphere at elevated temperatures. Common processes include pack cementation and noncontact vapor (gas phase) deposition techniques, or by diffusing corrosion-resistant species deposited by chemical vapor deposition (CVD) or slurry coating.
In pack cementation and noncontact vapor deposition techniques, vapor of the desired corrosion-resistant coating species (e.g., chromium, aluminum, etc.) is generated and caused to contact surfaces on which the coating is desired. The vapor reacts with the surface to deposit the desired coating specie(s), which are then diffused into the surface through a heat treatment. Aluminide diffusion coatings deposited by pack cementation or noncontact vapor deposition are often preferred for turbine blade airfoils. The dovetails of turbine blades are typically machined prior to the diffusion coating process, and may be masked during coating so that the dovetail will properly assemble with the dovetail slot in the rotor during engine build. However, during engine operation the under-platform regions of the blade can become corroded. In the past, corrosion of under-platform regions of turbine blades has been addressed by applying a vapor-phase chromide coating. While capable of improving corrosion resistance, vapor-phase chromizing processes require masking to prevent the chromide coating from being deposited on other surfaces of the blade, such as those already provided with an aluminide coating. However, masking is time-consuming, expensive, and not always effective.
Slurry processes generally entail the use of an aqueous or organic solvent slurry containing a volatile liquid vehicle and a powder of the corrosion-resistant coating specie(s) that can be sprayed or otherwise applied to a substrate, after which the substrate is heated to evaporate the volatile components of the slurry and, with further heating, diffuse the remaining coating species into the substrate. An example of a slurry composition is disclosed in U.S. Pat. No. 3,248,251 to Allen as containing aluminum particulates dispersed in an aqueous, acidic bonding solution that also contains metal chromate, dichromate or molybdate, and phosphate (the latter of which serves as a binder). The chromate ions are known to improve corrosion resistance. One prevalent theory described in U.S. Pat. No. 6,074,464 is that chromate ions passivate the bonding solution toward aluminum and inhibit the oxidation of metallic aluminum. In this manner, particulate aluminum can be combined with the bonding solution without undesirable reactions between the solution and aluminum. The coatings described in Allen are known to very effectively protect some types of metal substrates from oxidation and corrosion, particularly at high temperatures.
A drawback of slurry compositions of the type taught by Allen is the reliance on the presence of chromates, which are considered toxic. In particular, hexavalent chromium is considered to be a carcinogen. When compositions containing this form of chromium are used (e.g., in spray booths), special handling procedures closely followed to satisfy health and safety regulations can result in increased costs and decreased productivity. Therefore, attempts have been made to formulate slurry compositions which do not rely on the presence of chromates. For example, U.S. Pat. No. 6,150,033 describes chromate-free coating compositions used to protect metal substrates such as stainless steel. Many of the compositions disclosed in this patent are based on an aqueous phosphoric acid bonding solution, which comprises a source of magnesium, zinc, and borate ions. However, chromate-free slurry compositions can have various disadvantages, such as instability over the course of several hours (or even minutes), and generation of unsuitable levels of gases such as hydrogen. Furthermore, chromate-free slurry compositions have been known to thicken or partially solidify, rendering them very difficult to apply to a substrate by spray techniques. Moreover, the use of phosphoric acid in the compositions may also contribute to instability, especially if chromate compounds are not present since the latter apparently passivates the surfaces of the aluminum particles. In the absence of chromates, phosphoric acid may attack the metallic aluminum particles in the slurry composition, rendering the composition thermally and physically unstable. At best, such a slurry composition will be difficult to store and apply to a substrate.