Elevated Temperature Service and Corrosion Properties
Elevated Temperature and Corrosion Properties are often important criteria for selection of porous P/M materials versus other porous materials such as plastics and papers. The service environment may be oxidizing, reducing, or inert in nature and may contain many corrosive materials. Mechanical properties at an elevated temperature may be significantly reduced by accelerated corrosion reactions in atmospheres that contain even trace amounts of ammonia, carbon, chlorine, fluorine, hydrogen, moisture, sulfur, vanadium or other reactive materials. This Chart should be used as a guideline only since the actual maximum service temperature is highly dependent on specific environmental conditions. Product testing in the actual environment and operating temperature is highly recommended prior to service installation.
|Chart of Maximum Temperature Guidelines for General Service|
|Material||Maximum Temperature Oxidizing Atmosphere||Maximum Temperature Reducing Atmosphere|
|Bronze||150 °C (300 °F)||400 °C (750 °F)|
|316L SS||400 °C (750 °F)||540 °C (1000 °F)|
|Inconel TM 600||590 °C (1100 °F)||800 °C (1500 °F)|
|Hastelloy TM X||790 °C (1450 °F)||900 °C (1700 °F)|
Catastrophic oxidation of 316L SS can occur at a lower temperature than expected due to the formation of low melting point eutectic such as vanadium pentaoxide if vanadium is used as a reaction catalyst. Temperature fluctuations and localized hot spots can also cause accelerated oxidation due to spalling or cracking of the passive layer.
Corrosion resistance of porous P/M materials is a very complex subject due to the large number of variables which can change the material behavior. Material variables include composition, density, pore size, pore distribution and pore surface condition. Environmental variables include fluid composition, velocity, aeration, temperature, process cycle and exposure conditions. Corrosion failure of porous materials is normally due to localized attack of the sinter bonds due to inter-granular corrosion, crevice corrosion, stress corrosion cracking, and pitting mechanisms. Limited design engineering data is available to predict the corrosion rates of porous materials in various environments. In general, porous materials have significantly higher corrosion rates due to their larger surface area than the comparable wrought material. In order to predict the service life of porous materials without actual testing, corrosion handbooks for solid materials offer a reference point. As general rule, avoid selecting a porous material for an environment which does rate the corrosion resistance of the solid material as excellent. Even a very low corrosion rate for a solid material at the specified conditions can often still be too high for a porous material which obtains its mechanical properties from the very small bonds at the particle contact points. The corrosion resistance of a solid is often rated using a low fluid velocity in contact with the surface layer. Increased fluid velocity in the pore structure and aeration of the fluid may decrease the corrosion resistance of a porous material if the passive layer on each powder particle is disturbed. Also, pitting and crevice corrosion mechanisms can occur in porous materials which have dead end pore channels. Pitting corrosion can also occur if a corrosive liquid remains in the pores to create a localized concentration cell. Inadequate cleaning or prolonged storage without drying after removal from service can cause pitting failure.
There are many methods to enhance the corrosion resistance of porous materials. Proper alloy selection will result in the best protection against corrosion although the economic considerations of the more expensive alloys may be prohibitive. One advantage of the P/M process is that special alloys or custom blends can be made. For example, more nickel, chromium and manganese can be added to improve corrosion resistance of a 300 series stainless steel rather than being limited to standard alloys. Bronze filters can be plated with nickel or tin to improve corrosion resistance. If porous parts are welded, then a secondary annealing or stress relieving heat treatment can increase corrosion resistance and mechanical properties. Proper handling of the porous parts to minimize contamination may also improve corrosion resistance.