ABSTRACT
Of the corrosion-resistant, wrought nickel alloys, those containing significant additions of both chromium and molybdenum are the most versatile. Chromium promotes the passivation of such alloys in the presence of oxygen, and molybdenum ennobles the alloys under active corrosion conditions. Major attributes of these Ni-Cr-Mo alloys include high resistance to hydrochloric and sulfuric acids, and outstanding resistance to chloride-induced localized attack. Despite their versatility, there is a need within the chemical process and pharmaceutical industries for materials with even higher resistance to hydrochloric and sulfuric acids. This need is partially satisfied by the binary nickelmolybdenum alloys; however, these cannot tolerate even small quantities of dissolved oxygen. This paper describes a new, wrought nickel alloy, much more resistant to hydrochloric and sulfuric acids than the Ni-Cr-Mo materials, yet tolerant of dissolved oxygen and other oxidizing species. Furthermore, the alloy is extremely resistant to chloride-induced pitting and crevice attack.
INTRODUCTION
The corrosion-resistant, wrought nickel alloys are critical materials of construction for the chemical and pharmaceutical industries. Commercially pure nickel, for example, is the material of choice for caustic environments, the nickel-copper (Ni-Cu) alloys are among the few materials capable of withstanding hot, wet hydrofluoric acid, and the nickel-molybdenum (Ni-Mo) alloys excel in the two most important industrial chemicals, hydrochloric acid (HCl) and sulfuric acid (H2SO4), provided no oxidants are present.1-4 The most versatile and widely used corrosion-resistant nickel alloys are those containing chromium and molybdenum (the Ni-Cr-Mo alloys). Together, these elements provide resistance to a wide spectrum of industrial chemicals, in particular chloride salt solutions, which can induce pitting, crevice attack, and stress corrosion cracking of lesser materials. They are not as resistant to HCl and H2SO4 as the Ni-Mo alloys, but can tolerate impurities and additions of an oxidizing nature, such as dissolved oxygen. The importance of hydrochloric and sulfuric acids to the chemical process industries cannot be overstated. Thus there has always been a desire for materials with improved resistance to these two acids, without the drawbacks associated with the nickel-molybdenum alloys. In this paper, such a material is described, and its characteristics are defined. It is significantly more resistant to HCl and H2SO4 than the Ni-Cr-Mo alloys, surprisingly resistant to oxidizing chemicals, and possesses exceptional resistance to pitting, crevice attack and stress corrosion cracking, in the presence of chloride salt solutions.
ALLOY DESIGN
The nominal composition of the new material, which is named HASTELLOY® HYBRID-BC1 alloy, is given in Table 1, along with the nominal compositions of existing corrosion-resistant nickel alloys containing high molybdenum and chromium contents. The unique properties of this new alloy were discovered during autoclave tests of several commercial and experimental materials in 2.5% hydrochloric acid at 121°C (250°F). When purged with nitrogen, this solution is strongly reducing. When purged with oxygen, it is strongly oxidizing. As expected, the Ni-Mo alloys exhibited high resistance to the nitrogen-purged solution, but corroded rapidly in the oxygen-purged solution. The reverse was true for some of the Ni-Cr-Mo materials (N06022, N06200, and N06059), while others (N06455, N10276, N06686, and N06210) corroded rapidly in both the nitrogen-purged and oxygen-purged solutions.
