What Does Measure of Alloying for Resistance to Corrosion (MARC) Mean?
Measure of alloying for resistance to corrosion (MARC) is a concept used to measure the susceptibility of different alloys to corrosion. It refers to the following equation:
MARC = Cr (%) + 3.3 Mo (%) + 20 C (%) + 20 N (%) – 0.5 Mn (%) – 0.25 Ni (%)
From the MARC equation, the susceptibility of stainless steels to the localized corrosion of austenitic in chloride-containing solutions can be measured. MARC performance is far better than the pitting resistance equivalent (PRE) equation in indication of the corrosion susceptibility of alloys, especially when alloys are high nitrogen stainless steels. The MARC equations only work when alloying elements such as nitrogen, and only as long as they are in a solid solution.
Corrosionpedia Explains Measure of Alloying for Resistance to Corrosion (MARC)
Pitting resistance equivalent (PRE) is a well-accepted ranking of alloys in terms of their pitting addiction. The PRE equation is:
PRE = Cr (%) + 3.3 Mo (%) + xN (%), whereas x = 13…30
The value of x = 16 for steels containing Mo < 4,5 % and x = 30 for steels containing Mo 4,5 – 7,0 %.
Though PRE takes Cr, Mo and N into account, it does not do so for any other elements. Therefore, a new formula to include C, Mn and Ni into the PRE equation has been suggested. This equation is MARC.
It is the first formula that considers carbon to be beneficial against pitting. Some prefer a high carbon + nitrogen alloyed steel for higher pitting resistance. The higher thermodynamic stability of Cr-Mn-NC systems than carbon-free Cr-Mn-N steels caused improved pitting resistance. For caution, both carbon and nitrogen must not form any precipitations, although they remain as solid solutions.
In localized corrosion, the MARC formula indicates the positive effect of nitrogen, whereas manganese and nickel have a negative influence on pitting and crevice corrosion.