(Corrosion Division Morris Cohen Graduate Student Award) In-situ Monitoring of Ultra-slow Oxide Growth on Copper
Authors: Hosseinpour Saman , Schwind Markus , Langhammer Christoph , Kasemo Bengt Herbert , Johnson Magnus C , Leygraf Christofer 
Published: February 2020

The atmospheric corrosion of metals and alloys is a very complex process, which involves a wide range of chemical, electrochemical, and physical processes occurring in gas, liquid, and solid phases, as well as at the interfaces between them. Essentially all metals and alloys undergo corrosion in indoor or outdoor atmospheric conditions due to the presence of gaseous corrosion stimulators. However, the rate of the atmospheric corrosion might vary significantly depending on the nature of the metals or alloys, the concentration of corrosive species, as well as the exposure conditions. In some applications such as electrical contacts in microelectronics, military equipment or nuclear waste containers, even ultra-slow oxidation/corrosion may result in catastrophic consequences. In order to study such slow oxidation/corrosion processes, techniques involving ultra-high vacuum (UHV) are commonly used. Despite a very high surface sensitivity of these techniques they suffer from the drawback that the experiments are not performed under normal ambient conditions, but rather under vacuum. This has the consequence that the water adlayer which always exists on metal surfaces in atmospheric environments is removed under UHV conditions. Furthermore, in the case of ex-situ UHV studies, the structure of the oxidation/corrosion products might undergo vacuum induced changes. Hence, in-situ and highly surface sensitive techniques capable of working under atmospheric pressures are desired to monitor very slow oxidation and corrosion processes on metals and alloys at a molecular level.
In the present study, several analytical techniques have been used to obtain a picture as complete as possible of an atmospheric corrosion process, focusing on both the formation of corrosion products and the evolution of the protective layer. Copper surfaces covered with protective organic monolayers have been exposed to various atmospheric environments with the aim of simulating typical indoor atmospheric corrosion processes. Infrared reflection/absorption spectroscopy (IRAS) was used to determine the nature of the corrosion products, quartz crystal microbalance (QCM) to measure the mass of them, and nano plasmonic sensing (NPS) to determine the corrosion rate as well as to evaluate how homogeneous the formation of the corrosion products was. In addition, an inherently surface sensitive technique, vibrational sum frequency spectroscopy (VSFS) was used to study induced changes in the molecular structure of the protective monolayers.
VSFS studies of octadecanethiol (ODT) covered copper surfaces exposed to dry air allowed the detection of the formation of thin copper oxide layers and the observation of accompanying changes in the molecular structure of the hydrocarbon chains of ODT. VSFS was combined with QCM to quantify the amount of copper oxide formed on the surface. Since oxidation and corrosion processes often occur heterogeneously, the VSFS-QCM setup was integrated with NPS, which enabled us to distinguish homogeneous and heterogeneous corrosion under in-situ conditions with a time resolution on the order of seconds. This unique combination of three in-situ techniques facilitates the study of different corrosion processes where interfacial changes in electronic and optical properties as well as changes in mass occur. The sensitivity of the techniques under current exposure conditions corresponds to 5% of an oxide monolayer for VSFS, 2.5% for QCM, and 1.2% for NPS.
As an extension of the above mentioned studies a combination of VSFS and IRAS was used to compare the corrosion protection efficiency of alkanethiol SAMs with different chain lengths and to compare SAMs with –S or –Se as their anchoring group when the samples were exposed to humidified air (HA) containing formic acid (FA). It was found that the corrosion inhibition efficiency of alkanethiol SAMs on copper was enhanced with an increased chain length. Furthermore, alkanethiol SAMs feature “selective hindrance” properties, i.e. they selectively allow the penetration of formic acid and oxygen and hinder the penetration of water resulting in the formation of copper formate and copper hydroxide. Interestingly, no formation of copper oxide was observed, which was in contrast to unprotected copper. VSFS revealed that SAMs of alkanethiols get more disordered over the course of sample exposure to HA+FA but remain attached to the substrate. Alkaneselenol SAMs initially behave similarly to their alkanethiol counterparts. However, after prolonged exposure to HA+FA they are partially removed from the surface, resulting in an accelerated corrosion of the copper substrate. Eventually, the corrosion products formed on alkaneselenol SAM covered samples consists of copper formate, copper hydroxide, and copper oxide.

Figure 1