free-content
HomeFree Content Corrosion Reports Combatting...  

Combatting Liquid Metal Attack by Mercury in Ethylene and Cryogenic Gas PlantsTask 1 - Non-Destructive Testing


Abstract:
Presented herein are the results from the Task 1 nondestructive testing studies conducted under the a research program entitled, "Combatting Liquid Metal Attack of Aluminum Alloys by Mercury in Ethylene and Cryogenic Gas Plants". This program was organized and conducted by CLI International, Inc., under industrial multiclient support. The goal of the program was to develop preventative and remedial measures and detection and monitoring methods for mercury (Hg) attack of Al-alloys commonly used in the ethylene and cryogenic gas plant service.
My Saved Article

INTRODUCTION

Presented herein are the results from the Task 1 nondestructive testing studies conducted under the a research program entitled, "Combatting Liquid Metal Attack of Aluminum Alloys by Mercury in Ethylene and Cryogenic Gas Plants". This program was organized and conducted by CLI International, Inc., under industrial multiclient support. The goal of the program was to develop preventative and remedial measures and detection and monitoring methods for mercury (Hg) attack of Al-alloys commonly used in the ethylene and cryogenic gas plant service.

This report summarizes the results of tests conducted in Task 1 of the abovementioned program. It also serves to document experimental and analytical procedures used in the nondestructive testing of Al specimens exposed to Hg-containing environments under various loading configurations.

Laboratory tests were conducted to evaluate the acoustical emission (AE) from Hg-contaminated and control (Hg-free) specimens of Al-alloy 5083-0. These tests were conducted on compact tension (CT) specimens under either dead weight and hydraulic loading and on welded pipe specimens under internal pressure. Both types of specimens showed evidence of corrosive attack on fresh metal surfaces and liquid metal embrittlement (LME) in the presence of Hg. Computerized AE monitoring equipment was utilized to record and analyze the AE response of these specimens under various conditions of loading and pressurization. These conditions included increasing and decreasing load (pressure) as well as hold periods at constant load or pressure.

Characteristic AE response from both control and Hg-contaminated specimens was obtained. The control specimens were found to have increasing AE with rising load (pressure) with rapid attenuation during periods of constant load. High amplitude (>50 dB) AE from these specimens showed the Kaiser effect. Hg-contaminated specimens shoved a more complicated AE response than the control specimens. The response from contaminated specimens was characterized by a substantial increase in low amplitude (<50 dB) AE over the control specimens which appeared to occur readily at low pressures (0 to 500 psi) in pipe specimens and at low loads in CT specimens. Prolonged high amplitude (>50 dB) AE was observed in most cases during hold periods at constant load. While substantial low amplitude AE was found to occur at loads prior to the previous load maximum, this behavior was not completely characteristic of the Felicity effect.

BACKGROUND

Acoustic emission (AE) testing has many natural advantages for locating damage associated with Hg attack in Al-alloys used for cryogenic gas service. It is non-intrusive, rapid and can potentially locate defects from remote sensors. Therefore, if the characteristic AE associated with Hg attack of Al-alloys can be defined in laboratory tests, better and more complete inspection of process equipment used in ethylene plant service may be performed.

Liquid Metal Attack of Al-Alloys

In many characteristics, LME is similar to other environmentally induced cracking phenomena such as hydrogen embrittlement cracking (HEC) and stress corrosion cracking (SCC). As shown in Figure 1, environmentally induced cracking often involves both chemical or electrochemical and mechanical processes which result in the initiation of a crack of subcritical dimensions. It then propagates until it reaches a critical size and thereby causes fracture of the material. [1] In LME, as in SCC, breakdown of the protective surface film is a critical stage in this process. Al-alloys are protected from corrosion under normal conditions by their aluminum oxide surface film. However, chemical attack as well as mechanical forces alone or in combination can work to break down this passive oxide layer.

Not all liquid metals result in LME of metal substrates. As shown in Figure 2, it is related to the solubility of the liquid metal into the substrate material along with its ability to reduce the fracture surface energy. Consequently, solid-liquid metal couples can be categorized as shown in this figure. [1]

One very important aspect of LME is that typically once a LME crack is initiated, crack propagation rates tend to be very rapid (see Figure 3). [2] Hence lies the serious concern with using AE test methods for the detection of LME. AE field or in-plant test methods usually involve evaluation the vessel or component at a stress or pressure higher than those found during its normal operating conditions. Since LME may result in rapid, uncontrollable propagation of brittle cracks, it is important to know if AE can obtain meaningful information at low enough stresses or pressures where catastrophic failure of the vessel or component is not a major concern.

The present study attempts to evaluate the use of AE techniques to identify LME of Al-alloys produced by exposure of specimens to Hg under various conditions of stress and pressure. The experimental methods employed involve AE monitoring of stressed fracture mechanics specimens and pressurized vessels made from welded pipe exposed to both inert and Hg contaminated environments in the laboratory.

Acoustic Emission Testing

When metals are exposed to a corrosive environment, several processes can occur which result in detectable AE (see Figure 4). [3] These chemical processes involve anodic and cathodic reactions and film rupture as well as mechanical processes such as plastic deformation (i.e. dislocation motion and twinning), crack propagation, if residual or applied stresses are present, and phase transformations. Typically, the AE from chemical sources are of low amplitude and do not contribute greatly to overall AE during plant and field inspections. Only in cases where extensive corrosion product formation, gas generation and spalling occur do corrosion processes themselves play an important role in AE inspection.

More importantly, the combination of corrosion and mechanical processes can produce crack initiation and propagation events typical of HEC, SCC and LME in susceptible materials. These generally would be expected to produce high amplitude AE which can be readily observed in AE inspection. Consequently, AE inspection of plant vessels to identify SCC damage is fairly routine. [4]

AE has been employed to evaluate Al and Al-alloys in a number of applications. Typically, in non-corrosive systems, tensile straining causes AE to increase dramatically at the beginning of plastic deformation by the production and movement of dislocations corresponding to the onset of macroscopic yielding. The AE will then decrease somewhat with subsequent plastic deformation (see Figure 5). [5,6] Upon further straining, AE can be directly linked to crack initiation and growth processes. Typically, AE amplitude increases with the grain size of the material. [7]

Under conditions of corrosion, AE has been found to be useful in the evaluation of crack growth by either intergranular or transgranular SCC in metals. [6-10] However, as shown in Table 1, intergranular SCC and HEC typically provide higher amplitude and more easily monitored AE response than transgranular SSC. [6] Studies have shown that for intergranular SCC, the primary source of AE during crack growth was from the failure of ligaments behind the advancing crack front. [9] This probably results in highly localized deformation of the material which provides high AE intensity. It has also been observed that AE associated with SCC in Al-alloys is characterized by the correlation between AE activity and crack velocity as shown in Figure 6. [10]

Interesting effects have also been found in material systems where HEC is operating. [11,12] Specimens under cathodic charging were found to exhibit AE bursts which were related to the formation of hydrogen cracking in the material. Tensile testing conducted on specimens following hydrogen charging showed increased AE over uncharged specimens due to the effects of internal hydrogen on the deformation processes and possible cracking produced in the material during charging. Additionally, when these specimens were unloaded and loaded a second time, the uncharged specimens did not exhibit significant AE until reaching the stress used in the first cycle. This behavior is referred to as the Kaiser effect. However, the hydrogen charged specimens, exhibited AE at stresses below that used in the first cycle. This lack of Kaiser effect is referred to as a Felicity effect. The Felicity effect has been attributed to the formation of subcritical cracks in the lattice in this case caused by the hydrogen in the lattice and the stress from the first cycle.

TECHNICAL APPROACH

The experimental program in the present study was developed to determine the ability of AE to detect LME in Al-alloys. Four series of tests were performed as shown below:

Series 1 -

Evaluation of Compact Tension Specimens for AE Response in Hg.

Series 2 -

Evaluation of Pipe Specimens for AE with Internal Pressure and Hg.

Series 3 -

Evaluation of Pipe Specimens for AE Using Pressure Cycles and Hg.

Series 4 -

Evaluation of Kaiser/Felicity Effects with Compact Tension and Pipe Specimens with Hg.

These tests started in a very exploratory manner to observe the AE response of both compact tension (CT) fracture mechanics specimens and pressurized pipe specimens in both inert and Hg-contaminated environments. Once the exploratory tests were completed, a more detailed engineering study was conducted which tried to characterize the AE response from LME and determine the practical feasibility of using AE as a nondestructive testing tool in the evaluation of Al-alloy components in ethylene plant service.

EXPERIMENTAL PROCEDURE

Materials and Specimens

The alloy used in this study was alloy 5083-0. The chemical composition is provided in Table 2 along with its nominal mechanical properties. Al-5083-0 is commonly used in the construction of cryogenic equipment in ethylene plant service.

Pipe and plate material was obtained for this study and was evaluated in both welded and non-welded condition. Welding procedures were typical of those used in fabricating plant equipment.

The 1T compact tension (CT) specimens were machined per ASTM E-399 to the dimensions given in Figure 7. A nominal specimen thickness of 0.25 inch was used for these specimens. Side grooves were machined in the specimens in an attempt to enforce planar crack growth and test validity. Both welded and non-welded CT specimens were employed in this program. The welded specimens were oriented so that the crack would be growing in the weld metal parallel to the direction of the weld.

Pipe specimens were fabricated from nominal 5-inch O.D. pipe. To fabricate the pipe samples, sections of pipe were butt welded together using a full penetration single V weld. Samples were tested both with and without aluminum backing rings on the inside of the pipe at the weldment. As shown in Figure 8, the pipe specimens were machined on the ends to provide a true O.D. on which the pressurizing fixture could seal.

Test Methods

A total of four CT specimens were tested: Two non-welded and two welded. The apparatus shown in Figure 9 was used which provided a dead weight loading of the CT specimens. The specimens were exposed to increasing load in steps of 20 to 25 pounds or 5 to 10 pounds applied through pin and clevis grips. At certain load levels Hg was added to the notch area.

A complete report detailing the Series 1 AE test procedures and results is given in Appendix I.

Series 2 - Evaluation of Pipe Specimens for AE with Internal Pressure and Hg

Pipe specimens were monitored using the same type of AE sensors and instrumentation described in Series 1. For these tests, however, two sensors were used. The test set-up used in Series 2 is shown in Figure 10. It included the pipe specimen and end fixture which allowed for internal pressurization without end constraint. The AE sensors were placed on either side of the butt weld. The pipe specimen and sensors were placed in a plastic containment vessel to prevent contamination of the laboratory with Hg during failure of the pipe specimen. A manually operated, air driven hydraulic pump was used to hydrostatically pressurize the specimens to failure.

The specimens were pressurized with either oil or water both with and without Hg and with and without backing rings inside the pipe. For the Hg tests, a total of 1875 grams of Hg were added to the inside of the pipe sample prior to sealing. The Hg settled to the bottom (6 O'clock) position in the pipe during testing. The pressurized specimens were monitored using AE both during pressurization and during selected hold periods at constant pressure. The specimens were then pressured to failure.

A complete report detailing the Series 2 AE pipe test procedures and results is given in Appendix II.

Series 3 - Evaluation of Pipe Specimens for AE Usincr Pressure Cycles and Hg

The pipe specimens in Series 3 were tested using the same AE sensor configuration used in Series 2 except that a linear array of four sensors were used. These sensors were labeled 1 through 4 and were located along the length of the specimen. Two sensors were on one side of the butt weld and two were on the other side of the weld. Additionally, the containment and pressurization equipment were also the same as described previously in the Series 2 tests.

The main difference in Series 2 and 3 tests were in the pressure cycles employed. The pipe specimens were initially pressured to 450 psi for tan minutes. This pressure corresponded to approximately the normal working pressure of the Al pipe specimens. After this initial hold, the pressure was varied and held according to the schedule given below:

Step
1
2
3
4
5
6

Pressure (gsi)
500
525
450
500
525
550

Hold Time (min)
10
30
10
10
10
30

The pressures used in this sequence corresponded to approximately 110 and 120 percent of the original working pressure.

Following the pressure/hold sequence, the pipe specimens were subjected to rapid cyclic pressurization at one or more pressure levels in an attempt to initiate liquid metal attack in the pipe specimen. The initial pressure sequence was then repeated again followed by pressurization of the pipe specimens to failure. Pipe specimens were tested both with and without Hg contamination. The Hg tests were conducted with approximately 1000 grams Hg on the inside of the pipe.

A MONPAC severity analysis was conducted on the AE data. A complete report from detailing the Series 3 AE pipe test procedures and results is given in Appendix III.

Series 4 - Evaluation of Kaiser/Felicity Effects with Compact Tension and Pipe Specimens with Hg

Series 4 AE tests were conducted using the same AE instrument settings and similar laboratory techniques developed in the previous three series of tests. Series 4 tests involved experiments on both CT and pipe specimens using multiple load or pressure cycles. The typical cycle configuration consisted of a ramp to a designated load or pressure followed by a hold period until the AE signal subsided. Then, the load or pressure was decreased to approximately one half the previous maximum value and increasing again to a higher value of load or pressure. This sequence was continued until failure of the specimen was obtained.

During these load and pressure cycles, careful monitoring of the AE response of the specimen was conducted to observe whether Kaiser or Felicity effects were present. The presence of a Felicity effect could be evidence of LME attack of the specimen. Additionally, specimens were tested both with an inert environment (no Hg) and with Hg contamination. This allowed for a direct comparison of AE from the Al alloy both with and without the presence of Hg. The purpose of these observations was to determine specifically if an enhanced AE is obtained from the Al alloy when exposed to Hg as opposed to the same specimen tested in the inert environment.

A MONPAC severity analysis was conducted on the AE data. A complete report from detailing the Series 4 CT and pipe test procedures and results is given in Appendix IV.


1 2 3  Top
 

ABOUT SSL CERTIFICATES