Materials - Efects & Economic impact of CorrosionWhat is meant by Corrosion Allowance?By knowing the expected general corrosion rate and the anticipated plant or service life of a part, the designer can calculate the extra thickness required for corrosion resistance of the process equipment being designed.After determining a wall thickness that meets mechanical requirements, such as pressure, temperature and weight of equipment, an extra thickness called "corrosion allowance" is added to the wall thickness to comensate for the metal expected to be lost over the life of the equipment. Then, because the penetration depth cab very, a corrosion allowance is assigned a safety factor of two.Example:A tank wall required amm wall thickness for mechanical considerations. The designer has determined that the corrosion rate will be !." mm#yr and the expected life of the tank will be $!yr. The total corrosion allowance is the corrosion rate per year %!." mm x $! yr & " mm'.The corrosion allowance is doubled to ( mm as a safety consideration.Calculation of Corrosion Allowance)orrosion allowance is not only determined by the designer, but also and especially by astate or local agency. The latter often have years of experience with local conditions andespecially the weather conditions in a relevant area.*umidity, temperature, rain, wind, impurities and metal wet times have an effect on thecorrosion rate. )orrosion occurs when the relative humidity of the air is +! to (!,. )orrosion reaction is possible generally when the temperature is above !-) and the relative humidity is over (!, %the surface is wet'. Air impurities that dissolve in condensed water or rain water may accelerate corrosion. .ettling of dust and dirt on themetal surface accelerates atmospheric corrosion.)orrosion rates are expressed in terms of mm per year of surface wastage and are usedto provide a corrosion allowance in the design thickness of equipment such as vessels and pipework./perators will often use data based on historical experience from plant operations to aidthem in determining appropriate corrosion allowances. Alternatively corrosion charts are widely available that give corrosion rates for many combinations of materials of construction and process fluids and normally a range of values will be provided for various process temperatures.0n some instances, particularly where there is a mixture of chemicals present, appropriate data may not exist and corrosion tests may be necessary in order to determine the suitability of equipment. /perators should be able to demonstrate the use of corrosion allowances in equipment specification and design. The sources of data used should be traceable.As far as 0 know, there is no corrosion allowance exactly specified in A.12 B3$.3. )orrosion allowances are normally established by the end user and are somewhat basedon personal preferences and industry tradition. $. mm for piping is a common standard, but you are free to set a corrosion allowances you wish, unless a state or local agency has adopted and superceded B3$.3. To specify the pipe, add the corrosion allowance to the minimum design thickness and select a pipe schedule that is equal to or greater than the minimum 4 corrosion allowance.Below are two tables with ui!elines for corrosion allowanceCorrosion allowance for steel pipes mm Notes: For pipes passing through tanks, an additional corrosion allowance is to be considered in order to account for the external corrosion. Note 2: The corrosion allowance of pipes efficiently protected against corrosion may be reduced by no more than 50%. Note 3: hen the corrosion resistance of alloy steels is ade!uately demonstrated, the corrosion allowance maybe disregarded."uperheated steam 0.#"aturated steam 0.$"team coils in cargo tanks and li!uid fuel tanks %.0Feed water for boilers in open circuit systems &.5Feed water for boilers in closed circuit systems 0.5'low(down systems for boilers &.5)ompressed air &.0*ydraulic oil 0.#+ubricating oil 0.#Fuel oil &.0Thermal oil &.0Fresh water 0.$"ea water #.0,efrigerants referred to in "ection 0.#)argo systems for oil tankers %.0)argo systems for ships carrying li!uefied gases 0.#Corrosion allowance for non-ferrous metal pipes mm Notes: The corrosion allowance for other materials will be specially considered by the "ociety. here their resistance to corrosion is ade!uately demonstrated, the corrosion allowance may be disregarded. Note 2: -n cases of media with high corrosi.e action, a higher corrosion allowance may be re!uired by the "ociety.)opper 0,$'rass 0,$)opper(tin alloys 0,$)opper(nickel alloys with less than &0% of /i 0,$)opper(nickel alloys with at least &0% of /i 0,50luminium and aluminium alloys 0,5Corrosion Allowance eneralThe corrosion allowance is a measure which is used in the maintenance applications.A corrosion allowance of 3.! mm means that e.g. a wall thickness without any problem may be decrease with 3.! mm.1ind you, that does not mean than the minimum wall thickness is achieved and the ob5ect has reached its end of life.2xample6 a piece of equipment has a wall thickness of $! mm with a corrosion allowance of 3.! mm. 7ith a wall thickness of +.! mm action is required because the specified corrosion allowance is completely consumed..ome attention points when the corrosion allowance almost of completely is consumed. 7hat is the corrosion rate. 0s it linear or progressive occurred. 7hat was the wall thickness during construction. Are there baseline measurements are available. Are the process conditions changed. 0s there erosion occurred in combination with corrosion. )an we get the next inspection period. 7hat is the structural wall thickness that is needed. .hould we make a recalculation. 7all thickness monitoring during process %8reventive control'. 1ust the equipment to be replaced at the next shut9down. 1ust be the same material used for eventual replacement. The wall thickness measurements are reliable.8aint9and temperature can affect the measurement results. 0s the decline occurred internal or external, or a combination of both. )an equipement remain in use or should it be taken out of service.