ABSTRACT

Currently, evaporator tubes used to concentrate "wet process" phosphoric acid are made from high-chromium stainless steels and high-chromium nickel-iron alloys. While these offer good resistance to this highly contaminated chemical, improved materials are sought by the agrichemical industry, in the hope of extended use of evaporator tubes.

The objectives during the development of this new alloy were enhanced resistance to "wet process" phosphoric acid, high resistance to chloride-induced pitting and crevice attack (since the chloride content of this acid is usually high), and high thermal stability. These were attained with a simple ternary system, essentially Ni-33Cr-8Mo. Due in part to its high thermal stability, no problems were encountered in making production quantities of plates, sheets, bars, wires, welded tubes, and seamless tubes.

In this paper, the experimental findings and the corrosion characteristics of the chosen composition are defined. In particular, its advantages in "wet process" phosphoric acid and its resistance to localized attack in a ferric chloride solution are discussed.

INTRODUCTION

"Wet process" phosphoric acid, which is made by reacting phosphate rock with sulfuric acid, is one of the most important industrial chemicals, being the primary source of phosphorus for agrichemical fertilizers. As produced, it contains many impurities, and has a P205 concentration of only about 30 wt.%, because of the large amount of rinse water needed to separate it from the other main reaction product, calcium sulfate. Typical impurities include unreacted sulfuric acid, various metallic ions, fluoride ions, and chloride ions. The fluoride ions tend to form complexes with the metallic ions, and are therefore less of a problem than the chloride ions, which strongly influence electrochemical reactions between "wet process" phosphoric acid and metallic materials. Particulate matter (for example, silica particles) can also be present in "wet process" acid.

The main use of metallic materials is in the concentration process, where the "wet process" acid is taken through a series of evaporation steps, involving metallic tubing. Typically, the P205 concentration is raised to 54 wt.% during this process, while some companies concentrate the acid still further, to about 70 wt.% P205. The concentration effect upon the corrosivity of the acid is somewhat offset by the fact that the impurity levels drop as the concentration increases.

The metallic materials with the highest resistance to "wet process" phosphoric acid are the iron-nickel and nickel-iron alloys with high chromium and moderate molybdenum contents, notably N08028, N08031, and N06030. ~-3 The nominal compositions of these alloys are given in Table 1. All three alloys exhibit a face-centered cubic (austenitic) structure. The fact that a high chromium content is necessary indicates that "wet process" phosphoric acid is strongly oxidizing. It is likely that the moderate molybdenum contents prevent chloride-induced phenomena, such as pitting and under-deposit corrosion (a form of crevice attack).

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