Acoustic Emission From Simple And Stress Assisted Corrosion Reactions
Authors: Narasimhan S.L. , Blank George , Gottlieb Theodore 
Published: December 2007

Abstract
The continuing need to improve techniques to monitor and predict corrosion of oil well equipment prompted this study. It is shown that acoustic prompted this study. It is shown that acoustic emission (AE); (1) monitors the entry and effusion of hydrogen in steel, and (2) detects susceptibility to intergranular and stress corrosion cracking of various grades of steel.

Introduction
The spontaneous release of elastic energy during deformation or a phase transition of a material has generally been recognized as acoustic emission. Techniques based on the monitoring and analysis of acoustic emission signals have been extensively used in nondestructive testing and evaluation, and in deformation studies, especially relevant to fracture mechanics and hydrogen embrittlement. It is therefore logical to extend the AE technique to monitor cracking of materials under stress corrosion conditions for steels and stainless steels, as demonstrated by Okada et al. Rettig and Felsen reported the observation of AE arising from simple chemical and galvanic corrosion of steel and aluminum. More recently a detailed study of the chemical and electrochemical corrosion of steels in acid media revealed a substantial contribution to AE from the entry of hydrogen into steel, and suggested that the AE technique may provide a means of monitoring hydrogen assisted cracking of oil well equipment. Accordingly, it was the purpose of this study to analyze the acoustic emissions arising from steels under the following conditions:Simple chemical corrosion involving hydrogenentry,Electrochemical corrosion involving hydrogenentry,Intergranular corrosion of stainless steels,Stress corrosion of stainless steels,Sulfide stress cracking of steels.
Such an analysis was expected to provide guidelines for distinguishing between stress independent and stress assisted mechanisms contributing to the overall stress corrosion behavior of metals.

TEST PROCEDURES
The AE signals recorded in this work were detected by a commercially available system outlined in Figure 1. A band pass filter of 0.1–0.3 MHZ and a 50 db level were used after preamplification. The detecting transducer was attached to the sample by means of a constant force spring. For electrochemical corrosion studies a platinum wire electrode was employed as an anode. For platinum wire electrode was employed as an anode. For stress corrosion experiments the samples in the form of bars 0.18M (7") long, .012M (0.5") wide and 0.003M (0.125") thick were strained in a standard 3 point bending fixture. The fixture was made of 316 stainless steel and did not contribute to acoustic emissions in any of the corrosive media employed in this work.
Commercially available grades of 1020 and 4130 steel, and types 304 and 347 stainless steels were employed in this study. The 4130 steel was water quenched from 1550 deg. F and tested in the following conditions:
CONDITION HARDNESS YIELD STRENGHT-PSI
As Quenched 53 230,000 Q plus 1175 deg.F/30 min 23 10,000 Q plus 825 deg.F/30 min 38 175,000 Q plus 625 deg.F/30 min 43 200,000

RESULTS AND DISCUSSION

A. Simple and Electrochemical Corrosion
Consistent with the observations of Rettig and Felsen, considerable acoustic emission was recorded during the simple chemical corrosion reaction of steel in hydrochloric acid. The rate of emission is observed to be directly proportional to the surface area of steel exposed to the acid, as illustrated by Figure 2.
H+H+2e H2 gas (Recombination reaction)