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

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).

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

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