Failure Investigation and Materials Selection Analysis on Copper Pipes on a Distillation Condenser

Introduction

The condenser tubes on a gas cooler used in a whisky distillation plant have shown extensive corrosion over a 5 year period resulting in failure of the condenser.

Investigation

One Eighty determined the mode of corrosion failure and selected alternative copper alloy tubing for extended life of the condenser. The failed condenser was inspected.  The copper piping shows corrosion along a length 30-40 mm away from the tube sheet. There is no significant corrosion of the stainless steel.  In some cases severe corrosion has occurred at the tube sheet interface on the copper pipes.  It was also reported that the condenser stands vertically, and the corrosion observed was found at the bottom of the condenser.  There is a small reservoir at the bottom of the condenser at the tube sheet/pipe interface. This results in a small volume of medium collecting on the tube sheet surface. SONY DSC Figure 1 – Stainless Steel /Tube Sheet interface Compositional analysis revealed that the pipes are 99.9% pure copper. Energy Dispersive X-Ray Analysis (EDS) was conducted on the corrosion product and indicates that it is rich in sulphur (Figure 2). 2 (Custom)

Figure 2: Energy dispersive x-ray analysis result on corrosion product

What We Found

Results

Three corrosion mechanisms are possible:
  • Uniform attack due to presence of sulphur
  • Crevice corrosion at the tube sheet interface due to reservoir build up
  • Dissimilar metal corrosion.
Since corrosion has occurred a long a distance away from the tube sheet, the presence of sulphur is the most likely corrosion mechanism. Copper alloys and stainless steels are relatively close to each other in the galvanic series. Therefore the contribution of this failure mechanism is probably minimal. The concentration of sulphur compounds is greater at the bottom end of the condenser.  Therefore, although crevice corrosion would have played a significant role, the role of uniform corrosion by means of sulphur compound attack is more significant.  Materials Selection The distillation process is optimised if the vapours are distilled against copper.  Consequently, a copper alloy pipe must be selected that shows better resistance to corrosion in a sulphur compound bearing environment. No specific standards could be found for distillation equipment. However, ASTM B111/B111M-09 gives a standard specification for copper and copper alloy seamless condenser tubes and ferrule stock.  On the basis of this standard, a subset of copper alloys was selected for further investigation. The performance of these recommended alloys is shown in Table 1. Performance in various environments was accessed and ascribed a rank, A (recommended) B (acceptable) and C (not recommended).  It can be seen that only the aluminium bronzes are recommended for all the various sulphur containing environments (Table 1).  The specific aluminium bronzes recommended are C60800, C61300 and C61400.  Additional data was found for carbon dioxide as well as black, white and green liquors. The aluminium bronzes are recommended or acceptable for these applications.  Thus overall, the aluminium bronzes present an attractive alternative to pure copper for the application. Consideration was given to the use of copper alloys rather than pure copper for the distillation application.  The NSF/ANSI 51 – Standard for food and beverage materials was consulted and no exclusions were given for copper alloys for the use in food and beverage applications.  Consequently there are no known restrictions to the use of aluminium bronze for the distillation equipment application.   Table 1 – List of ASTM B 111 recommended alloys for condenser tubes

Pure Cu

C10100

C10200

C10300

C10800

P Bronze

C12000

C12200

C14200

C19200

Red Brass

C23000

Muntz Metal

C28000

Admiralty

Metals

C44300

C44400

C44500

Aluminium

Bronze

C60800

C61300

C61400

Nickel

Bronze

C70400

C70600

Solid S

C

N/A

C

C

C

A

C

SCl

C

N/A

C

C

C

C

C

SO2 -dry

A

N/A

A

A

A

A

A

SO2 - moist

A

N/A

C

C

C

A

C

SO3

A

N/A

A

A

A

A

A

H2SO478% or less

B

N/A

C

C

B

A

C

H2S dry

C

N/A

C

C

C

B

C

H2S moist

C

N/A

C

C

C

B

C

CO2 dry

A

N/A

A

A

A

A

A

CO2 moist

B

N/A

C

C

B

A

A

Liquors-Black

B

N/A

B

B

B

B

C

Liquors-Green

C

N/A

C

C

C

B

C

Liquors-White

C

N/A

C

C

C

A

C

Conclusion

The primary corrosion mechanism that has caused the diminution of the copper pipes in the condenser is uniform corrosion as a result of sulphur bearing compound attack. Crevice corrosion mechanisms cannot be ignored especially since a small reservoir exists at the tube sheet of the condenser. Although a small variation exists in the galvanic potential for stainless steel and copper alloys, the contribution of this mechanism to the diminution of the pipes is probably minimal. A range of copper alloys have been considered as alternative materials for the condenser tubes and the aluminium bronzes have been found to be the most suited to resist diminution in a sulphur compound bearing environment.  Thus it is recommended that the copper pipes be replaced with a suitable aluminium bronze material as listed in ASTM B 111/B111M-09 standard specification for copper and copper alloy seamless condenser tubes and ferrule stock.

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