INTRODUCTION
ABSTRACT The corrosion behavior of high-performance austenitic stainless steels and nickel-based alloys in hydrofluoric acid at room temperature was investigated. For alloys with Ni contents between 20 and 50% the corrosion rate can be described by general equations for aerated and deaerated hydrofluoric acid. The corrosion rate can be calculated considering the hydrofluoric acid concentration, the presence of air and the alloy content of Cr, Ni, Mo and Cu. These equations reflect a primary beneficial effect of Cr and Mo and a limited positive effect of Ni in aerated acid. In deaerated acid there is a primary beneficial effect of Ni, Mo and Cu and a limited positive effect of Cr. Thus the relative influence of the alloying elements can be described by an Alloy Equivalent Sum (AES) each for aerated and deaerated acid. The equations facilitate the selection of economic materials for HF-containing equiPment at ambient temperature. Potentiostatic current density versus potential diagrams indicate if an alloy is active or passive at the respective corrosion potential and thus explain the different effect of aeration on corrosion (positive or negative) depending on alloy composition
Hydrofluoric acid (HF) is used for a number of industrial products like:
Ø ~" Cooling fluids
Ø Pickling products
Ø ~" PTFE products for corrosion protection (e.g. PTFE liners, gaskets)
Ø Products for private usage (e.g. households).
The production capacity of HF in the NAFTA region was estimated with 434 thousand metric tons of anhydrous and aqueous hydrogen fluoride. The annual demand in 2001 was determined with 350 thousand metric tons and forecasted for 2005.1
Aqueous HF has to be catogorized as severely corrosive. Besides non-metallic corrosion production solutions various nickel-based alloys can be chosen as materials of construction for hydrofluoric acid service. Ni-Cu and Ni-Mo alloys are preferred under reducing conditions but especially Ni-Cu alloys exhibit high corrosion rates in the presence of oxygen. Ni-Cr-Mo alloys are useful under oxidizing conditions. High-performance stainless steels and iron-modified nickel-based alloys can be applied in a limited concentration and temperature range where they could be economic alternatives to high alloyed nickel-based alloys. A review of HF corrosion including an overview of industrial processes using HF and problems concerning materials selection has been published recently. 2'3.
The primary objective of the investigations reported here was to scientifically understand and quantify the corrosion behavior of high-performance stainless steels and iron-modified nickel-based alloys in HF and to elucidate the characteristic electrochemical effect of the presence of air. Parts of the experimental results have been presented before. 4
EXPERIMENTAL
Materials
Commercial stainless steels and nickel-based alloys were tested. Depending on the chemical composition the 21 different grades of alloys can be divided into the following five groups (Table 1): I (Fe-Cr-Ni-Mo), II (Fe-Ni-Cr-Mo-Cu / Fe-Ni-Cr-Cu-Mo), III (Ni-Fe-Cr-Mo-Cu), IV (Ni-Cr-Fe-Mo[-Cu]), and V (nickel-based alloys with Ni > 50% and Fe<10%). Main emphasis of the investigations has been on groups II, HI, IV of alloys with 20-50% Ni (high-performance stainless steels and iron-modified nickel-based alloys). The materials were tested in the mill-annealed condition. Coupons (40 x 15 mm, thickness 2 to 6 mm) were used for exposure tests and potentiostatic measurements. The coupons were provided with a hole for support and electrical contact. Measurements of the corrosion potentials and potentiodynamic experiments and were performed with coupons (10