Stanislaw Mrowec

Recent progress in corrosion-resistant metastable alloys

Tailoring new corrosion-resistant alloys has recently been performed mostly by the sputter deposition technique. This technique is suitable for forming a single-phase solid solution even when the boiling point of one component is lower than the melting points of the other components and/or when one component is immiscible with another component in the liquid state. Aluminium-refractory metal, chromium-valve metal and molybdenum-chromium-nickel alloys have been successfully prepared in a single amorphous phase. Amorphous aluminium-refractory metal alloys are corrosion resistant in 1 M HCl and chromium-valve metal alloys are spontaneously passive in 12 M HCl, showing a better corrosion resistance in comparison with the alloy components. The amorphous aluminium-refractory metal alloys also have an extraordinarily high hot corrosion resistance. Their sulphidation resistance at higher temperatures is far higher than any other known metallic materials and their oxidation resistance is comparable to chromia- or alumina-forming alloys.

Corrosion-resistant amorphous surface alloys

Abstract This is a review of laser and electron beam processing and sputter deposition for the preparation of corrosion-resistant amorphous surface alloys and of the characteristics of thus prepared surface alloys. Amorphous surface alloys with a large surface area were prepared by repetition of instantaneous melting of a very restricted volume of the surface by irradiation with a CO 2 laser or electron beam and subsequent self quenching by the cold bulk substrates. The materials consisting of the amorphous surface alloys and bulk crystalline metals are quite suitable for corrosion-resistant materials with other specific properties. Sputter deposition was used for preparation of various amorphous alloys such as Al-Ti, Al-Zr, Al-Nb, Al-Ta, Al-Cr, Al-Mo, Al-W, Cr-Ti, Cr-Zr, Cr-Nb, Cr-Ta, Cu-Nb and Cu-Ta. The corrosion resistance of amorphous aluminum alloys was very high and could be changed from that comparable to 304 stainless steels to that far exceeding corrosion-resistant nickel-base alloys by changing the alloying element and its concentration. The corrosion rates of amorphous chomium-valve metal alloys are several orders of magnitude lower than those of alloy constituting elements. The amorphous Al-Mo alloys show high resistance to both sulfidation and oxidation at high temperatures. Sputter deposition is a potential method to produce new materials with specific properties.

New amorphous alloys resistant to high temperature corrosion

Abstract High temperature corrosion of sputter-deposited amorphous Al(34–46)Mo and Al31Mo(6–16)Si alloys has been studied as a function of temperature (973–1273 K) in sulfur vapor (10 3 Pa), as well as in oxygen and air. The sulfidation process follows parabolic kinetics, so is diffusion controlled. Over the whole temperature range studied, the discussed alloys showed excellent resistance to sulfide corrosion, their sulfidation rates being comparable with the oxidation rates of chromia-forming materials. The oxidation resistance of binary AlMo alloys has been found to be satisfactory, but only at temperatures not exceeding 1073 K. Above the melting point of MoO 3 (1069 K), the scale, consisting mainly of Al 2 O 3 with about 5% MoO 3 , becomes unprotective because of the evaporation of MoO 3 . However, ternary AlMoSi alloys show excellent resistance to oxidizing environment up to about 1200 K, their oxidation rates being comparable with those of Al 2 O 3 formers. No traces of molybdenum have been found in the Al 2 O 3 scale on these alloys.

Kinetics methods in studying the transport properties of nonstoichiometric metal oxides and sulphides

Two kinetic methods for studying the transport properties of transition metal sulphides and oxides have been described. It has been shown that modern microthermogravimetric techniques enable, in quite a simple way, the determination of the concentration and the mobility of point defects in these materials as a function of temperature and oxidant activity, with an accuracy difficult to attain with other, much more complicated and time consuming methods. The advantages of the kinetic methods described in this paper have been illustrated by the results obtained on Mn - Mn 1-y S - S 2 system which has been studied in detail with other conventional techniques.

High temperature corrosion of sputter-deposited Al–Nb alloys

Sputter-deposited Al-Nb alloys have excellent sulfidation resistance, although their initial sulfidation rate is high due to the preferential sulfidation of aluminum. The preferential sulfidation of aluminum leads to the enrichment of niobium at the metal/scale interface, and consequently, the protective NbS2 scale is formed on the Al-Nb alloys. The sulfidation rate of the Al-Nb alloys is lower than that of niobium. The resistance of these alloys to high temperature oxidation is higher than that of niobium. The oxide scale formed on the Al-Nb alloys at high temperatures is composed of a double aluminumniobium oxide and alumina. Because of the formation of the less protective double oxide, the oxidation resistance of Al-Nb alloys is lower than that of typical alumina-forming alloys.

The Influence of Yttrium on High Temperature Oxidation of Valve Steels

Abstract The influence of small amounts of yttrium, electrochemically deposited on the surface of four steels utilized in the production of valves in car engines, on the protective properties of the oxide scale and its adherence to the surface of the oxidized materials has been studied under isothermal and thermal cycle conditions. Oxidation measurements have been carried out at 1173 K. It has been found that yttrium addition improves considerably the scale adherence to the substrate surface, increasing thereby corrosion resistance of the studied materials.

Corrosion Behavior of Valve Steels In Oxidizing Atmosphere Containing Acetic Acid

The oxidation kinetics of X33CrNiMn23-8 and X50CrMnNiNbN21-9 valve steels have been studied in oxidizing atmosphere, containing 49.5% of water vapor and 0.5% of acetic acid in the temperature range 10731273 K. It has been found that the mechanism of corrosion under these conditions is rather complex. After early stages of the reaction, lasting between 20 and 40 hours, the process follows approximately parabolic kinetics. It has been found that the presence of acetic acid highly increases corrosion rate of X33CrNiMn23-8 steel, containing rather high chromium content, but has no influence on the corrosion rate of X50CrMnNiNbN21-9 steel with lower chromium concentration.

Sputter-deposited amorphous Al–Mo–Si alloys resistant to high temperature sulfidation and oxidation

Sputter-deposited amorphous Al-(31–45)Mo-(6–21)Si ternary alloys were prepared to improve the oxidation resistance of the sulfidation-resistant Al-Mo binary alloys. The ternary alloys showed excellent oxidation resistance in air even at temperatures above the melting point of MoO3 (1069 K) when molybdenum content of the alloys was about 30at%. The oxidation rate of the Al-Mo-Si ternary alloys was almost comparable with those of typical alumina-forming materials. XPS analysis revealed that the addition of silicon depressed molybdenum oxidation and assisted the formation of alumina scale without molybdenum oxide on these alloys. The high oxidation resistance of the Al-Mo-Si alloys resulted from the fact that during the heating of the ternary alloys, in addition to the Al8Mo3 intermetallic compound, the silicon rich Mo5Si3, was formed instead of molybdenum rich AlMo3 formed in Al-Mo binary alloys. The Al-Mo-Si ternary alloy containing 6at% of silicon also showed the significantly high sulfidation resistance, but excess amount of silicon addition is detrimental for sulfidation resistance.