Corrosion phenomena in flowing media, illustrated for chemical pumps. It is shown how centrifugal pumps in chemical plant are exposed to a complex superposition of (static and dynamic) mechanical and thermal stresses, of corrosion, erosion/abrasion and cavitation. Then examples of pump parts that have been operated under service conditions are shown to demonstrate appearance and origin of flow‐induced, erosion‐ and cavitation‐corrosion. Materials concepts are discussed that are able to avoid or reduce these phenomena. Several pump‐like test methods are introduced that permit selection of pump materials in accordance with the practical service conditions, where hydraulic, design, and materials criteria have to be optimized to guarantee long pump lives.
Corrosion-and Erosion-Resistant Pump Material -
Investigations Under Flow Conditions Close to PracticeThe major goals of these investigations can be described as follows: -design, construction and test of laboratory apparatus to study the resistance of pump impellers under corrosive and erosive attack in various aggressive media,-simulation of flow conditions characteristic for pumps as close to practice as possible to study the non-uniform attack in corrosive media of various type, concentration and temperature. The following materials were used in these studies: -a femtic-austenitic cast stainless steel of type 0,03% C -24% Cr -6% Ni -2,5% Mo -3% Cu -0,15% N (German designation: G-X 3 CrNiMoCuN 24 6) -a purely austenitic cast stainless steel of type 0,07% C -25% Ni -20% Cr -3% Mo -2% Cu -0,25% Nb (German designation: G-X 7 NiCrMoCuNb25 20, Werkstoff-No. 1.4500). The investigations were concentrated on the corrosion and erosioncorrosion behaviour of these steels in static and flowing sulfuric acid of various concentrations and temperatures. The effect of flow conditions of the acid was studied by evaluating the weight losses of the materials in three different apparatus: the rotating disc, the oncethrough tube and the impeller with tube-shaped flow canals.The main result of these studies was the well exposed effect of flow conditions of the acid on the corrosion and erosion-corrosion behaviour of the materials. A detailed analysis shows that: 1. there is the well-known decrease of the corrosion resistance of both materials under flow in the rotating disc apparatus. 2. there is, however, an increase of the corrosion resistance in the tube and model impeller apparatus compared to static solutions. 3. there is a strong increase of the corrosion rate in the region of active corrosion itself under any flow condition of the acid. 4. there are three types of corrosive attack occuring on the duplex stainless steel independent of the flow of the sulfuric acid: -uniform corrosion,-selective corrosion of the femte, -selective corrosion of the austenite.5. there is a region of undefined corrosion behaviour separating the range of corrosion resistance from that of active corrosion for the duplex as well as for the austenitic stainless steel.
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