Lipid Layer-Based Corrosion Monitoring on Metal Substrates

This method could be a cost-effective, nondestructive platform for a broad range of materials analysis techniques.

Corrosion is a deforming process that costs the United States Department of Defense (DOD) approximately $23 billion annually and accounts for 23% of all DOD maintenance. Exhaustive efforts have been made towards the detection and diagnosis of this issue; however, the problem persists. The Army’s “Go Green” initiative has opened the door for research into an environmentally friendly, biologically based corrosion monitoring technique. For this reason, novel research is being conducted on the use of lipid layers in corrosion monitoring.

Comparison of Uncoated and Coated specimens after 14 days in a salt fog chamber (top), and (bottom) after 42 days in a salt fog chamber.

Lipids are a class of macromolecules that serve as the primary constituent of cell membranes and energy storage centers in living organisms. When phosphorylated, or bonded to a phosphate (PO4-) group, they become higher-energy am phiphilic molecules capable of forming a bilayer. In biological systems, this phospholipid bilayer works with protein complexes to regulate material transport, as well as to send and receive information between cells. While natural phospholipids function in living systems, advances in technology over the past few decades have given scientists the ability to study phospholipids in nonbiological laboratory settings.

This work explores the use of phospholipids in corrosion monitoring applications. When metal is oxidized, material is lost in the form of metal oxides. Phospholipids have been observed to become more conductive in the presence of certain chemicals, possibly disrupting the bilayer. If metal oxides are capable of disrupting and causing degradation of a phospholipid layer, it may be possible to detect and diagnose the extent to which the metal has corroded by measuring this degradation.

Spin-coating these synthetic lipids onto the metal substrate is a very simple process. It was originally speculated that a hydrophobic silica monolayer would be necessary before the lipids would adhere to the metal. They seem, however, to adhere effectively to the aluminum plates after slightly abrading the surface past the glossy finish and cleaning in an ultrasonic de-ionized water bath. For this experiment, the lipids 1, 2-dipalmitoyl-snglycero- 3-phosphocholine (DPPC) were purchased in chloroform solution. The lipids were used without further preparation. Sheet stock of aluminum alloy 6061 was cut into plates of 5 × 5 × 0.15 cm dimensions. The plates were cleaned with acetone and placed in an ultrasonic de-ionized water bath before experimentation. The lipids were applied by pipetting dropwise to a plate until covered, and then accelerating to rotation in a spin-coating machine at 1000 rpm for 60 seconds.

The plates were left to sit for 15 minutes to allow residual chloroform to evaporate from the lipids before any further testing was performed. The plates were then weighed and placed into a probe station to be observed at 100x magnification to assess the quality of the applied lipid film. The plates were placed in a salt fog chamber at constant temperature of 95 °F and let to corrode for 3, 6, and 7 days. Once taken out of the corrosive environment, the plates are weighed and placed in the probe station for further electrical resistance and conductivity testing.

The initial surface response to the salt spray chamber was monitored progressively using visual inspection prior to quantitative evaluation techniques. After 14 days, there does not exist on the macroscopic level any visible pitting or corrosionlike presence. Even after 42 days, not much evidence of corrosion was observed. The samples were then placed under plexiglass in the salt fog chamber for accelerated corrosion.

Upon corrosion development, quantification techniques were used to show that electrical resistivity increases with increasing corrosion. By creating a grid on the plates and taking readings at different positions, values could be obtained that are more representative of the sample as a whole. This is repeated at different time intervals during a stay in the corrosive environment.

Prospective methods to characterize lipid layer degradation may include mass spectrometry, atomic force mi croscopy, surface profilometry, and scanning electron microscopy. These methods have aspects that would be ideal for determining the surface topography and detecting flaws in the lipid layer on a submicron scale. The trend that accompanies lipid degradation data can be used alongside the trend in impedance data and used to estimate corrosion rate.

This work was done by Scott Kinlein, Anindya Ghoshal, and James Ayers of the Army Research Laboratory; and Daniel Cole of Motile Robotics, Inc. ARL-0156