HOT CORROSION

11.9 HOT CORROSION

Hot corrosion is the accelerated oxidation of a material at elevated temperatures induced by a thin fi lm of fused salt deposit [36] . It is called ‘ hot corrosion ’ because, being caused by a thin electrolyte fi lm, it shares some similarities with aqueous atmospheric corrosion, in which corrosion is commonly controlled by the diffu-

sion of oxygen to the metal surface. In hot corrosion, the soluble oxidant is SO 3 ( SO 2 2 − 7 ) in the fused salt. In the late 1960s, during the Viet Nam confl ict, hot corrosion caused severe corrosion of gas turbine engines of military aircraft during operation over sea- water. Sulfi de formation results from the reaction of the metallic substrate with

a thin fi lm of fused salt of sodium sulfate. The condensed liquid fi lm deposits

OXIDATION

either by chemical deposition (i.e., the vapor pressure of Na 2 SO 4 in the vapor phase exceeds its equilibrium partial pressure at the substrate temperature) or by physical deposition (solid or liquid salt detaches from an upstream component and attaches to a hot substrate on impact) [36] . In metallographically examining corroded components, oxide particles were observed to be dispersed in the adher- ent salt fi lm. As a result, a fl uxing mechanism may apply, in which an otherwise protective oxide scale on the substrate surface dissolves at the oxide/salt interface, but precipitates as non - protective particles within the salt fi lm [36] . The fl uxing mechanism of hot corrosion has been thoroughly reviewed by Rapp [36] .

Hot corrosion may occur in the absence of metallic contaminants in the fuel. It was fi rst identifi ed in marine propulsion gas turbines where service temperatures of fi rst - stage blades and vanes were in the range 650 – 700 ° C (1200 – 1300 ° F). The accelerated attack was repeated in the laboratory [37] by applying a thin coating

of Na 2 SO 4 to either 30% Cr – Co or 30% Cr – Ni alloys and oxidizing at 600 – 900 ° C (1110 – 1650 ° F) in O 2 at 1 atm containing 0.15% (SO 2 + SO 3 ). The highest oxidation rates occurred at 650 – 750 ° C, in accord with practical experience. Oxidation pro- ducts consisted mostly of once - liquid mixtures of Na 2 SO 4 + CoSO 4 (or NiSO 4 ). Metallic sulfi des often form inclusions or a network of sulfi des along grain bound- aries of the alloy; the corresponding attack is then called sulfi dation .

It has been suggested that lower oxidation rates above 750 ° C result from formation of protective Cr 2 O 3 fi lms unable to form at lower temperatures; e.g., at 650 ° C [37] . In turn, the presence of Cr 2 O 3 fi lms may be related to the increased volatility and lesser tendency of Na 2 SO 4 to deposit on the alloy surface at higher temperatures [38, 39] . Damaging deposits of Na 2 SO 4 may originate alone from sea salt contamina- tion of intake air. Sulfur dioxide and trioxide of oil combustion products also contribute, but hot corrosion of marine turbine blades may occur even with use of very low - sulfur fuels [38] . High - chromium alloys are more resistant to hot corrosion than are low - chromium alloys.

Ilschner - Gensch and Wagner suggested [27] that the mechanism is not one of melting point or fl uxing alone, but may also involve galvanic effects. A spongy, porous network of an electronically conducting oxide, such as Fe 3 O 4 fi lled with a liquid electrolyte, reproduces the cell described previously containing a platinum cathode and a nickel anode in contact with molten borax. The Fe 3 O 4 sponge acts as an oxygen electrode of large area, and the base metal acts as an anode. Supplied with a liquid electrolyte in which oxygen and metal ions migrate rapidly, such a cell accounts for an accelerated oxidation process far exceeding the rate for a metal reacting directly with gaseous oxygen through a continuous oxide scale.