In the manufacturing world, zinc coating is the most effective and widely used material to prevent corrosion, maintain the strength, and increase the longevity and performance of steel. We learned from a previous article that the quality of galvanized steel might depend on zinc coating weight and that sometimes it is difficult to differentiate between lead and zinc.
Zinc-coated steel substrates often have an additional organic coating applied, protecting the steel surface or altering its appearance for decorative reasons. These organic coatings are expected to have long lifetimes and since they are often used outside, they must be durable and have good weathering properties. The characteristics that make a particular organic coating suitable for steel protection will depend on the elemental and chemical composition of the organic material.
X-ray Photoelectron Spectroscopy (XPS) is an analysis technique that provides chemical bonding information from the top few nanometers of a surface down to many microns in depth. It is the ideal technique, for example, for identifying the carbon functional groups in an organic coating. XPS cannot detect hydrogen, however, but the complementary technique of Reflection Electron Energy Loss Spectroscopy (REELS) is able to both detect and quantify hydrogen. REELS can also be used to discern between aliphatic and aromatic carbon.
A multi-technique surface analysis system that combines XPS and REELS can be used to analyze an organic coating on steel, yielding a full elemental and chemical quantification, including hydrogen concentration. Hydrogen and Chemical Quantification of an Organic Coating details an experiment using such an instrument.
Wide scan XPS survey data identified all elements present, from the surface of the organic coating, down through its 25μm thickness and finishing in the zinc-coated steel substrate. Quantification of the XPS data provided an elemental profile of the sample (excluding hydrogen) as a function of depth and enabled study of the carbon bonding states.
High energy resolution carbon spectra were acquired at each point along a linescan, allowing the carbon bonding as a function of depth to be determined. The XPS elemental analysis identified two distinct regions in the organic coating, which were HMMM-rich and HMMM-depleted.
A REELS linescan was performed on the organic coating cross section, providing complementary information to the XPS analysis. REELS spectra were analyzed to quantify hydrogen and to investigate the change in aromaticity as a function of depth.
The strongest peak in a REELS spectrum is due to electrons elastically scattered from the sample. The small peak at the base of the elastic peak is due to electrons interacting with hydrogen atoms in the organic coating. Simple peak fitting of this shoulder versus the elastic peak enables quantification of the hydrogen versus all non-hydrogenic peaks. This data was combined with the XPS quantification to give a total elemental quantification throughout the coating.
High energy resolution XPS analysis of the carbon chemistry indicated that increased aromaticity in the HMMM-rich zone was not solely due to the HMMM cross-linker. REELS analysis of the organic coating confirmed this conclusion.
Read Hydrogen and Chemical Quantification of an Organic Coating for study details and spectra.