GI-00415, Rev. E August 2009 Micro Motion® Corrosion Guide For Coriolis Flow and Density Meters, Dedicated Density Meters, and Viscosity Meters H2SO4 HCl NaOH HNO3 C6H8O7 CH4 Cl2 H3PO4 C3H8O Disclaimer: The guidelines in this publication are provided for informational purposes only. Minor changes in fluid properties (e.g., temperature, concentration, impurity levels) can affect the compatibility of wetted parts. Material compatibility choices are solely the responsibility of the end user. © 2009 Micro Motion, Inc. All rights reserved. The Micro Motion and Emerson logos are trademarks and service marks of Emerson Electric Co. Micro Motion, ELITE, MVD, ProLink, MVD Direct Connect, and PlantWeb are marks of one of the Emerson Process Management family of companies. All other trademarks are property of their respective owners. Contents To satisfy the need of selecting the right material for a given application, Micro Motion manufactures meters in 316L, 304L, and super duplex stainless ® Coriolis flow and density meters . 1 steels, 316L stainless steel lined with Tefzel coating, nickel alloy C-22, titanium, and tantalum. Policy for mixed material bi-metallic meter compatibility. 6 Material compatibility How to use the material compatibility table for Coriolis meters . 9 Material compatibility must be considered in more Material compatibility table for detail for Coriolis meters as compared to Coriolis meters . 10 pressure-containing pipe. Compatibility in the latter case is usually addressed by consulting a Application notes . 32 general corrosion guide. General corrosion is a Dedicated Density and viscosity meters . 36 term that refers to the uniform loss of material. The rate of material removal is usually expressed How to use the material in terms of inches or millimeters lost per year. compatibility table for Dedicated These rates are determined experimentally by Density and viscosity meters . 38 exposing a sample to the environment for a Material compatibility table for specific time period. Weight loss or dimensional tube density meters . 39 changes are then used to determine the corrosion rate. Material compatibility table for fork density meters, viscosity meters . 61 General corrosion tests are insensitive to Synonyms . 91 detection of localized corrosion and are not always adequate for determining material compatibility for Coriolis meters. Pitting, intergranular attack, stress corrosion cracking, Coriolis flow and density meters and corrosion fatigue are all forms of localized corrosion that can lead to meter failure. Localized corrosion of the flow tube can initiate Coriolis mass flow and density meters are a major fatigue cracking. Meter failure can then occur due advance in flow measurement. These devices to the rapid rate at which fatigue cracks have set a precedent for accuracy and propagate. The approach to preventing meter repeatability under a wide variety of flow failure is to avoid the onset of fatigue cracks. For conditions. The inherent precision has established this reason, the possibility of localized corrosive it as a standard for numerous industrial attack must be considered when selecting wetted applications. The ability of these meters to materials. measure mass flow and density directly has led to their use in applications ranging from metering food products to corrosive chemicals. Coriolis meters have proven extremely reliable when metering noncorrosive fluids. The same reliability can be achieved in corrosive services if consideration is given to the compatibility of the process fluid with the sensor materials of construction. Micro Motion® Corrosion Guide 1 Coriolis flow and density meters continued Material compatibility cannot always be assessed Figure 1). Stainless steel can also be used in by considering the alloy(s) selected for the organic solutions that contain a chloride remainder of the piping system. Material component, provided ion formation is avoided. compatibility for most piping systems is based Two factors that influence dissociation are upon general corrosion rates alone and does not temperature and moisture. Both need to be kept account for localized corrosion or cyclic loading. low to avoid failure. Figure 2 shows that the Coriolis meters require vibration of one or two flow resistance of 316L to free chloride-induced tubes to make a mass flow or density corrosion fatigue is temperature dependent. Low measurement. The cyclic loading condition is combinations of temperature and chloride inherent to all Coriolis meters and must be concentration are compatible with 316L stainless considered in the material selection process. steel. Pitting and corrosion fatigue are possible for higher combinations of temperature and chloride Material compatibility variables concentrations. Alloy C-22 should be used when these conditions exist. If the chloride content is The numerous environments in which the meter increased further and pH lowered, alloy C-22 may can be used make it difficult to define process fluid also succumb to localized attack and corrosion compatibility for every possible material fatigue. combination. The difference in chemical composition of most environments can be pH characterized by four variables. These are halogen concentration, pH, chemical potential, The pH of a solution can also alter the corrosion and temperature. If these variables can be defined behavior of any given alloy. In general, solutions for a particular environment, comparisons of alloy that have a neutral pH (near 7) tend to be less limitations can be made and a compatible material aggressive than strongly acidic (pH < 3) or of construction chosen. Figures 1 through 4 show strongly alkaline (pH > 11) solutions (see the domain of acceptable performance for 316L Figure 3). Tantalum, for example, has superior stainless steel, alloy C-22, titanium, and tantalum corrosion resistance to 316L stainless steel and as a function of the first three variables. The effect alloy C-22 in neutral and acidic environments. of temperature on meter life can be characterized However, high corrosion rates will occur if by considering its effect on the other three tantalum is used in caustic applications such as variables. sodium hydroxide, even at room temperature. At higher temperatures, stress corrosion cracking Halogens and corrosion fatigue are possible. Under these conditions, alloy C-22 is recommended. Alloy The term halogen refers to a specific group of C-22 should be used in all caustic applications in elements and includes chlorine, fluorine, bromine, which there is a possibility of chloride and iodine. The most common halogen is contamination. chlorine. The presence of the ionic form, Cl¯, even as a contaminant, can be extremely detrimental to corrosion resistance. Stainless steels are particularly susceptible. Meters constructed of 316L stainless steel have been extremely reliable in numerous applications where chloride concentrations can be maintained at sufficiently low levels or where free chlorides are absent (see 2 Micro Motion® Corrosion Guide Coriolis flow and density meters continued Chemical potential The wide range of chemical potentials over which passivity is maintained make tantalum resistant to The chemical potential is a measure of the most corrosive fluids. The second most stable oxidizing or reducing power of a process fluid. oxide forms on the surface of nickel-based alloys Chemical potential, sometimes referred to as such as alloy C-22. A high chromium and redox potential, is defined relative to the molybdenum content stabilizes the oxide layer, H2 → 2H+ + 2e – half reaction, which is assigned a yielding improved performance over 316L value of zero volts. Any environment that has a stainless steel in chloride bearing applications. chemical potential greater than the reference is 316L stainless steel exhibits passivity over a considered oxidizing. Chemical potentials that are narrow range, as compared to the other two equal to or less than the reference are considered materials. However, 316L stainless steel has reducing. Chemical potential is important because proven to be suitable for a large number of a minimum amount of oxidizing power is required chemical processing applications. to enable the formation of protective surface oxide layers. Optimal life will be realized as long as this layer is stable. Environments that are too oxidizing or reducing will prevent stable oxide formation. Under such conditions, failure due to corrosion fatigue or erosion/corrosion is possible. The corrosion fatigue resistance of a material of construction is related to the range of chemical potentials over which oxide layer stability is maintained. The broader the range in Figure 4, the more environments in which the material will resist corrosion. Tantalum pentoxide (Ta2O5) is stable on the surface of metallic tantalum at extremely low reducing potentials. This oxide also resists breakdown in all but the most oxidizing environments. Micro Motion® Corrosion Guide 3 Figure 1. Typical chloride concentration Figure 2. Chloride ion concentrations and range for meter materials temperature limits for 316L High Temp. °C 60 316L unacceptable Use high-nickel-based alloy 50 40 30 Use 316L Low stainless steel 20 SS C-22 Titanium Tantalum 040 80 120 160 200 Chloride (ppm) Figure 3. Typical pH range for meter Figure 4. Chemical potential range for meter material materials High Reducing Neutral Low Oxidizing SS C-22 Titanium Tantalum SS C-22 Titanium Tantalum 4 Micro Motion® Corrosion Guide Coriolis flow and density meters