Debasis Jan 2, 2008 Corrosion Rate Half Life in RBI %E2%80“ a general view Your question is general again to give any specific information. However the following may give guidance on CRHL (Corrosion Rate Half Life). CRHL will differ in your own fluid process plant from one mechanical component to another %E2%80“ Ex: Static vessel components, furnace / heater components, piping components, compressor / turbine / pump components may differ in evaluation criteria and in CRHL values. CRHL will differ vastly from fluid process plant to other plants (Ex: Process refinery vs. nuclear plant life estimation) or from concrete re-bar corrosion rate in bridges etc. The basis is on selective corrosion rate analysis for generating an inspection and replacement schedule for each plant. The method involves steps to identify a system from a division of the plant into one or more system wherein each system has mechanical components that are expected to experience a common corrosion environment. For Ex: Take the case of simple UT thickness monitoring of a system. You have to perform at least one thickness measurement location in a system or sub-system to evaluate from CRHL. The next step is to establish system corrosion data for each thickness measurement location and running the data through a plurality of corrosion engineering models. Running the corrosion engineering models includes coordinating the system corrosion data of the thickness measurement locations into a plurality of physical relationship data groups and then applies a plurality of statistical distribution / goodness-of-fit tests to each physical relationship group. The final step is to select a best fitting corrosion engineering model for generating an inspection schedule for the fluid containment components within each system of the plant. You can choose to have a preferred embodiment of a method for selective corrosion rate analysis. The step of running a corrosion engineering models may include additional step of identifying sub-populations within the applied statistical distribution / goodness-of %E2%80“fit tests. Also you can choose to identify the appropriate sub-populations in assisting you (the user) to further identify un-known corrosion mechanisms (this could be SCC, fatigue, creep, etc) and establish specific inspection schedules and also choose additional further N.D.T techniques (other than conventional UTFlaw, UT Thickness, MPT, DPT, Visual etc) + choose other techniques like thermography, acoustic emission, imaging, in-situ metallography, creep testing etc to monitor each system or each sub-system populations or locations. Once established the user can derive corrosion rate to evaluate the half life or full remnant life in any of the system component chosen from inspection schedules and replacement plans. I suggest that for any further information on this subject (of interest to you), please go through RBI related published papers and books on specific components %E2%80“ piping, static vessel, turbine or compressor components, and concrete re-bar corrosion etc. You should recognize that CRHL will differ vastly from one plant to other %E2%80“ For ex: what is applied to a Refinery need not be the same for a nuclear industry system. Hope this helps you C.V.Srinivasan Nishi Engineers Pvt Ltd India Jan 2, 2008 E-mail: nishi@vsnl.com >In RBI , CRHL is used. Experts pl. discuss it's application >significance. >regards

Corrosion rate half line is a theoric concept which can be used only in piping systems where the main corrosion mechanism is expectable to be linear. In such a system after establish a corrosion rate it will be easy to define the CRHL. For localized corrosion mechanisms such as pitting, galvanic corrosion, erosion, or naphtenic acid corrosion it will be almost impossible to establish CRHL. In this case, to have an accurate corrosion monitoring control you must select very well your piping TML locations, and take profit of on line corrosion probes as well as UT thickness measurements based in safe periodic routines to avoid premature failures. Regards Luis Marques