Relative Humidity Cycling Effect on Ag and Cu Corrosion
Authors: Yuan Bo , Fleming Debbie A , Franey John , Opila Robert L. , Xu Chen 
Published: February 2020

Introduction

Corrosion-induced failures remain frequent in electronic equipment deployed in industrial and aggressive environments. ISA 71.04 standard [1] provides a classification system to evaluate the corrosive potential of an environment. Four levels of environmental severities (G1 Mild, G2 Moderate, G3 Harsh and GX Severe) are proposed based on Cu and Ag corrosion rates. Furthermore, ISA 71.04 standard posits that severity level can be increased by one level for a relative humidity (RH) fluctuation rate of change greater than 6% per hour. However, no experimental data is provided to validate this statement. Anecdotal evidence for the potential effect of RH cycling on the metal corrosion have been reported [2-3], where the corrosion rate in data centers with large humidity variations appears to be higher than data centers with small humidity variations. However, containment species and gas concentrations in these data centers were not monitored and controlled. Testing with well-controlled conditions is therefore needed to fully understand the role of RH fluctuation on metal corrosion rate.

Experimental

A mixed flowing gas (MFG) chamber was used to perform the RH cycling experiments. A three-way solenoid valve is added to the MFG system to produce RH fluctuations. Three cycling conditions were set: 52%-35%, 60%-38% and 70%-40%. The cycling time was set as 20 minutes and total exposure time in the chamber is 2 days for each experiment. The RH change rate as high as 180% per hour has been achieved in third condition. Corrosive gas concentrations are set close to field condition: H2S-50ppb, NO2-200ppb, Cl2-20ppb, SO2-100ppb. Constant RH (high limit and low limit RH) experiments were also conducted for comparison. The corrosion behavior of Ag and Cu are studied using different metrologies: weight gain, SEM/EDX, cathodic reduction (CR).

Results

For all three conditions, RH cycling did not increase the corrosion rate as measured by the weight gain of Ag and Cu coupons compared with the constant RH conditions. Based on cathodic reduction results, the main corrosion products for Cu and Ag are not changed under RH cycling. For Ag, the dominant product is Ag2S and for Cu, copper oxides and Cu2S are main corrosion products. SEM also shows no significant surface morphology change between the cycling condition and constant RH conditions. The corrosion on both Ag and Cu coupons is mostly determined by the level of the RH, rather than the cycling of the RH. The ISA 71.04 statement with respect to RH cycling therefore has been demonstrated to be invalid.
[1] ISA-71.04-2013 “Environmental Conditions for Process Measurement and Control Systems: Airborne Contaminants”
[2] Abbott, W. H., Natural and Laboratory Environmental Reactions on Materials and Components, 9th Progress Report, 1986.
[3] Singh, P., Klein, L., Agonafer D., Shah, J.M., Pujara, K.D., „Effect of Relative Humidity, Temperature and Gaseous and Particulate Contaminations on Information Technology Equipment Reliability”, Proceedings of the ASME 2015 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems and ASME 2015 13th International Conference on Nanochannels, Microchannels, and Minichannels InterPACKICNMM2015, July 6-9, 2015, San Francisco, California, USA