(414g) Pit Nucleation, Growth, and Coalescence: Modeling the Corrosive Effects of Mg Exposed to a 3.5% NaCl Aqueous Solution

The use of lightweight metallic alloys, such as cast magnesium, has recently gained traction in the automobile industry [1, 2]. The low density (1.74 g/cm3), high mechanical stiffness, excellent castability, and easy machinability make cast magnesium a strong candidate for structural components that are intricate and lightweight [2]. However, these benefits are greatly affected by a high corrosion rate when compared to aluminum or steel. In addition to a relatively high corrosion rate, magnesium has a high electrochemical potential, causing magnesium to corrode easily in the presence of seawater [3]. The high electrochemical potential of magnesium allows for coupon dissolution when magnesium is placed in a NaCl aqueous solution. The high corrosion rate has relegated the structural use of magnesium to areas that are not exposed to the environment, including car seats and internal electronic boxes [4, 5]. Therefore, the use of cast magnesium as a structural material requires a detailed understanding of the metal's response to corrosion conditions. Modeling of the pits present on the magnesium (Mg) surface due to corrosion should develop the desired understanding.

In the Center for Advanced Vehicular Systems (CAVS) at Mississippi State University, the electrochemistry between a conductor, Mg, and an electrolyte, sodium chloride, is being investigated. The total corrosion of two different types of Mg coupons, AE44 and AM60, was determined after exposure to a 3.5% NaCl aqueous solution. The total corrosion is determined based on the following model [6]:

Φ_Total = Φ_GC + Φ_IC

where

Φ_Total is the total damage from corrosion arising from any type of corrosion mechanism

Φ_GC is the damage from general corrosion (loss of thickness)

Φ_IC is the damage from pitting

To calculate the corrosion due to pitting, the following model was developed [6]:

Φ_IC = h_pn_pc

where

h_p is the pit number density related to nucleation of pits (number per unit area or volume)

n_p is the area of pit growth related to growth of the pits

and c is a function of the nearest neighbor diameter related to the coalescence of the pits

Following the initial exposure to the NaCl aqueous solution, changes in the surface resulted in changes to the nucleation, growth, and coalescence properties, as the dominant corrosion mechanism changed [7, 8]. Initially, the dominant form of corrosion was pitting, demonstrated by changes in the pit number density [7, 8]. During an interim period, intergranular corrosion and pitting corrosion were equal mechanisms, demonstrated by changes in both the pit number density and the growth of the pits [7, 8]. Finally, towards the end of the experiments, general corrosion became dominant, as demonstrated by changes in the growth of the pits and the nearest neighbor radius [7, 8]. While the nucleation, growth, and coalescence of was studies using AE44 and AM60, both surfaces were polished, while the corrosion effects of 3.5% NaCl on cast AE44 Mg and AM60 Mg have not previously been studied. Two different methods of salt corrosion were investigated, including constant immersion and salt-spraying. The research presented will cover the effects of immersion and salt-spray on the validity of the model developed to detail the corrosion of magnesium.

[1] Jambor, A.; Beyer, M. Mater. Des. 18 (1997) 203-209.

[2] Froes, F.H.; Eliezer, D.; Aghion, E.L. JOM. 50 (1998) 30-34.

[3] Shaw, B.A. Corrosion Resistance of Magnesium Alloys, in: L.J. Korb, ASM (Eds.), ASM Handbook, Vol. 13A: Corrosion, Ninth Ed., ASM International Handbook Committee, Metals Park, 2003, 692.

[4] Makar, G.L.; Kruger, J. Int. Mater. Rev. 38 (3) (1993) 138-153.

[5] Song, G.; Atrens, A. Adv. Eng. Mater. 5 (12) (2003) 837-858.

[6] Horstemeyer, M.F.; Lathrop, J.; Gokhale, A.M.; Dighe, M. Theor. Appl. Fract. Mech. 33 (2000) 31-47.

[7] Martin, H.J.; Alvarez, R.B.; Horstemeyer, M.F.; Chandler, M.Q.; Williams, N.; Wang, P.T.; Ruiz, A. Metall. Trans. A. In Review.

[8] Alvarez, R.B.; Horstemeyer, M.F.; Chandler, M.Q.; Williams, N.; Wang, P.T.; Ruiz, A.; Martin, H.J. Metall. Trans. A. In Review.