Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.
Issue link: http://mp.epubxp.com/i/804522
20 APRIL 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 4 MATERIAL MATTERS Continued f rom page 19 The electrochemistry of all corrosion, Rose and Legg explain, involves simulta- neous metal dissolution in one location coupled with oxygen reduction and hydrogen evolution in another location. The corrosion rate ty pically is inf luenced by the equilibrium, or balance, between the opposing electrochemical reactions. The location of the equilibrium point is determined by the galvanic current and voltage at which the cathodic current (oxygen reduction/hydrogen evolution) is equal to the anodic current (metal disso- lution/pitting). These currents and volt- ages are governed by the current-voltage characteristics of the surface beneath the electroly te—that is, the surface's polar- ization behavior. In practical terms, when the polarization curve is plotted in the standard "absolute current vs. potential" graph, equilibrium is the point where the cathode and anode curves cross. The software f inds this crossing point automatically. Rose adds that if the software only found the equilibrium point using data supplied by the user, then it would not be particularly useful because the user would still need to f ind the relevant polarization curves in the literature; hope they were measured in a similar, robust manner; and then enter the data into the application, all of which is time consum- ing and adds to the appeal of just looking up galvanic potentials on a table. What makes the software fast and accurate is its electronic polarization database. Whereas the older galvanic series tables provide the galvanic potentials of various generic materials, Rose says, the new digital electrochemical database provides the complete current-voltage characteristics of specif ic alloys, carbon f iber composites, coatings, passivates, and other surface treatments for various service environments, as well as new lightweight alloys and composites that are increasingly taking the place of steels and other alloys in modern aircraft, vehi- cles, and machinery. It is constantly being expanded and updated for curated mate- rials based on high-quality electrochemi- cal data measured in a consistent manner following a protocol developed by the Sea- Based Aviation team using best practices. The measurement protocol is very impor- tant, note Legg and Rose, because compu- tational methods require the galvanic potential (open circuit potential), as well as the entire current-voltage polarization curve for a surface and electroly te, to be well def ined. Currently, all of the data in the data- base are determined using an electroly te solution of 3.5% sodium chloride (NaCl); however, corrosion behavior under any electroly te, from deionized water to acids and alkalis, can be added. The data also make it possible to predict the self-corro- sion rate of alloys, as well as the galvanic corrosion rate between them. The software tool is designed to make the modeling approach accessible to engi- neers and designers who are not CA E experts. It can quick ly calculate the cor- rosion current and corrosion rate using the digital electrochemical database developed by the Sea-Based Aviation proj- ect, and enables the user, on the f ly, to predict corrosion rates and assess the corrosion risk for material interfaces. The user chooses a combination of two mate- rials, coatings, or surface treatments from the database, presses a button, and the resulting galvanic current density (corrosion rate) and mixed potential are displayed. To minimize corrosion, substi- tute materials/coatings/treatments can be selected and results compared to f ind the most cost-effective and safest option with the lowest corrosion rate. The evalu- ation takes about two minutes, which is less time than it takes to f ind the data in a table, and far less time than creating an electrochemical FEA model and inputting all the relevant data. Presently the software is being used on a test basis by several aerospace and general manufacturing companies. It is particularly useful for determining which material option can best replace sacrif i- cial coatings such as zinc and cadmium in existing products, as well as evaluating