The Attenuation of Galvanic Corrosion Between Mechanically-Coupled Aluminum and Polymer Matrix Composites (PMCs) Using Hydrophobic, Insulative Skirt Materials

                                                                                      OBJECTIVE

The objective of this research is to study the effect of hydrophobic, insulative skirt materials on the galvanic corrosion of mechanically-coupled aluminum and polymer matrix composites (PMCs)to develop effective corrosion mitigation solutions.

INTRODUCTION

Metal-PMC structures are being used more and more in military and civilian applications. PMCs, for example, are being used with greater frequency in defense, aerospace, automotive and civil-infrastructure industries. In the quest to reduce weight and increase performance, more and more advanced structures, systems and machines are being developed using a combination of alloys, composites, polymers, and ceramics. A system of dissimilar materials, however, may induce significant corrosion problems. Metal alloys coupled to composites may lead to crevice and galvanic corrosion problems. Corrosion control strategies need to be incorporated into the design of mechanically coupled metal-PMC joints to improve corrosion resistance and decrease maintenance and repair time. 

BACKGROUND

Electrically conductive PMCs such as carbon fiber reinforced (CFR) PMC can accelerate the corrosion at PMC-Al interfaces by galvanic action and crevice corrosion. A typical method used to mitigate galvanic corrosion when bolting aluminum to CFR PMCs is to insert an insulating skirt between the members (Figure 1) to eliminate direct contact. However, aluminum and CFR PMCs are still galvanically coupled through the fastener; hence, galvanic corrosion can occur if the electrolyte bridges over the insulating skirt. To attenuate the galvanic effect, the insulating skirt can be extended beyond the CFR PMC coupon to various lengths (Figure 1). 

APPROACH

The galvanic corrosion rate is a function of Ohmic loss (IR loss) between Al (anode) and PMC (cathode), and the polarization behavior of the anode and cathode. The Ohmic loss depends on the ionic conductivity of the electrolyte film that forms on the insulating skirt. The parameters that affect the ionic conductivity of the electrolyte film are salt loading (m’) (e.g., various Cl^- concentrations), length of the insulating skirt (l_s), and humidity. A relationship between galvanic corrosion rate and skirt length-to-salt loading ratio (l_s/m’) was established.

Another parameter that affects the effective skirt length to attenuate galvanic corrosion is the continuity of the electrolyte layer on the skirt. Hence, hydrophobicity of the skirt material is an important parameter in determining the effective skirt length. The insulating skirts of various lengths were coated with different coatings (Figure 2) and the hydrophobicity was determined using contact angle measurements.

The galvanic corrosion rate of 6061-T6 Al alloy coupled to CFR PMC were monitored over a 3-day period inside of a humidity chamber at 90% relative humidity (RH) and 30^oC using the zero resistance ammeter (ZRA) technique with different skirt lengths which were coated and uncoated. The samples were sprayed with various concentrations of salt solutions. The galvanic corrosion rates were monitored for different types of coatings on the insulating skirt (hydrophobicity), electrolyte type, humidity, and various skirt lengths. Measurements were also be taken in the field.

 ACKNOWLEDGEMENTS

The authors are grateful for the support from the Office of the Under Secretary of Defense for the project entitled “Corrosion and Corrosion Control Studies of Aluminum Alloys that are Mechanically-Coupled or Adhesively-Bonded to Polymer-Matrix Composites in Diverse Micro-Climates” (U.S. Air Force Academy, USAFA–BAA-2009-1). The authors are particularly grateful to Mr. Daniel Dunmire, Director, Corrosion Policy and Oversight, Office of the Under Secretary of Defense. The authors are also grateful to other members of the Department of Defense Technical Corrosion Collaboration:  Mr. Richard Kinzie, Mr. Larry Lee, Dr. William Abbott, Dr. David Robertson, Mr. Richard Hays, and Dr. Christopher Scurlock.