Abstract: Ion-plated oxide coatings for protection against corrosion

Abstract

The corrosive environment at high pressures and temperatures in coal gasifiers impose severe requirements on the metals and alloys of fabrication.1 The hostile conditions occur in the gasifiers, transfer lines, and other on-line components where a mixture of corrosive gases exists up to 2000 °F at a pressure between 100 and 1500 psig. Generally, gasifiers are lines with ceramic refractories to withstand high-temperature corrosive and erosive attack from the gases and particulates produced during gasification. Ceramic coatings are being investigated for similar purposes, especially for smaller on-line components. Various techniques are being explored to produce adherent and dense ceramic coatings. This work includes the application of thick (10–100 μm) aluminum oxide coatings via an ion-plating method. The ion-plating technique was first described and used by Mattox.2 It has been successfully used by other investigators3,4 to ion plate metals and alloys. In this technique, the material is evaporated via resistance heating,3 electron-beam impingement,2 or induction heating,4 then ionized and accelerated through the discharge, and finally deposited on the substrate. In the present work, the evaporation of material was achieved by using an i-Gun ion source developed by White.4 The i-Gun is an induction-heated evaporation source that serves to vaporize and ionize the material. Power is supplied by a rf generator at 450 kHz. Two coating structures were investigated, one with an intermediate metal bonding layer between a Type 304 stainless steel substrate and the aluminum oxide and the other without an intermediate layer. An insulating material, such as Al2O3, was ion plated using a susceptive graphite crucible or a resistance heat source. The normal rf-sputtering method was employed to deposit Al or Hf metals after a 15-min sputter cleaning. The rf supply, capable of maintaining 2000 V at 1 A, was used to bias the substrate and to provide predeposition sputter-etch cleaning. The aluminum oxide was deposited in a vacuum of ∠10−5 Torr (1.3×10−3 Pa). Water vapor constituted ∠85% (∠8.5×10−6 Torr, 11×10−4 Pa) of the background gas during deposition. The substrate was rf biased at ∠1200 V at a temperature between 300° and 350 °C during ion plating. To achieve evaporation and ionization of Al2O3, more power was required than for Al or Hf. As a result, depostion rates of Al2O3 as high as 12 μm/min were obtained. Some Al2O3 depostions were carried out at an O2 residual gas pressure of 5×10−5 Torr (6.6×10−3 Pa) to control stoichiometry. All substrate-coating systems (304SS–Al2O3, 304SS–Al–Al2O3, and 304 SS–Hf–Al2O3) that have aluminum oxide thicknesses ranging from 25 to 100 μm showed an excellent bending tenacity. The observed Knoop hardness of the coatings varied between 300 and 400 kg/mm2. The thermal-spalling resistance of the aluminum oxide coating on type 304 stainless steel was poor. This resistance was markedly improved when an intermediate layer of Al or Hf was employed. The corrosion resistance of the coating was evaluated in a coal gas mixture at 980 °C (∠1800 °F) for a period of 100 h. The protection of type 304 stainless steel by an aluminum oxide coating without an intermediate layer was observed to be poor because of the spalling. However, an intermediate layer of Al of Hf of appropriate thickness markedly improved the protection of the substrate.