Tag Archives: Zinc

Zinc Ribbon or Copper for AC Mitigation – that is the question…

Team members of MATCOR have been involved in several online discussions (LinkedIn) about the pros and cons of Zinc Ribbon for use in AC Mitigation. So we thought we would share our expertise in this subject on our blog.

Below are 4 reasons why we believe Copper (and our AC Mitigation product, “The Mitigator“) is a superior solution to Zinc Ribbon.

  • Formation of passive films on the surface of the zinc can cause a significant electropositive shift in the zinc potential over a period of time; this generally occurs over a period of days or weeks. The general rule of thumb is that the concentration of chlorides and sulfates must be measurably greater than the sum of the concentrations of bicarbonates, carbonates, nitrates and phosphates; otherwise with time the zinc corrosion potential will shift electropositive. Plattline’s Web site notes that zinc ribbon is “generally used with gypsum backfill”; however, too often for AC Mitigation applications, no consideration is given to placing the zinc ribbon in a specially prepared backfill (this should be general practice).
  • Zinc faces high consumption/corrosion rates in the presence of AC. A.W. Peabody has noted that AC can “create an especially high corrosion rate with buried aluminum, magnesium and zinc”. Testing of zinc electrodes at an AC Current density of 155 A/m2 found a 15-20 fold increase in the consumption rate of zinc. R.A. Gummow, a corrosion engineer and a NACE International accredited Corrosion Specialist, notes that “accelerated corrosion of zinc ribbon AC mitigation facilities must be expected and needs to be accounted for in the cathodic protection design despite the lack of information on the magnitude of the accelerating effect”.
  • The effect on existing impressed current CP systems: the use of zinc anodes directly connected to the pipeline for AC mitigation can interfere with existing impressed current CP systems in a way that is both difficult to model and to predict. In some cases, the zinc anodes can become an additional load, particularly if the zinc is not located in a prepared backfill and has shifted to a more electropositive potential. In other cases, the zinc anode may be providing and/or supplementing galvanic current to the CP system in which case it will be consumed over time – note that the presence of AC often increases significantly the consumption rate. This could result in premature consumption of the zinc ribbon as an AC Mitigation system.
  • The effect of the zinc ribbon on potential surveys when directly connected to the pipeline can be erratic and difficult to interpret, rendering these surveys inconclusive or invalid. Given the emphasis on integrity management and the additional risks posed by AC Induced Corrosion in collocated right-of-way (ROW) corridors, the negative impact that the zinc ribbon might have on survey data could make CIS surveys invalid and increase the need for and frequency of Inline Inspections (ILI).

In addition to these 4 key reasons, MATCOR’s ‘The Mitigator‘ is the pipeline industry’s first engineered AC Mitigation grounded system, with greater ease of installation and lower overall cost.

Galveston’s tall ship Elissa no longer seaworthy…corrosion issue

GALVESTON, Texas — The official tall ship of Texas is in trouble.

The iron and steel bottom of the three-masted 1877 Elissa is nearly rusted through in places, prompting the U.S. Coast Guard to declare that the vessel is not seaworthy.

Officials at the Texas Seaport Museum in Galveston where the Elissa is berthed were astonished when a Coast Guard inspection earlier this year revealed the rotten hull.

The tall ship is inspected twice every five years, said John Schaumburg, museum assistant director. The latest inspection uncovered the worst rot since the tall ship was rebuilt in 1982, he said.

Very little corrosion was discovered during the previous dry dock in 2008, prompting surprise that the bottom could have deteriorated so quickly, Schaumburg said.

“Everyone’s jaw just dropped,” said Ed Green, one of about 100 volunteer crew members from Houston. About half of the volunteers are from Houston, as are most of the ship’s visitors, Schaumburg said.

No one knows for sure what caused the rapid deterioration, but officials suspect that Hurricane Ike might be the culprit. Elissa rode out the September 2008 storm at a special mooring designed for violent storms, losing a sail, a spar and suffering some other minor damage.

The worst damage was unseen, Schaumburg said. Sea water eats into any metal, so 15 zinc “anodes” are fastened to the hull to draw off the corrosion. Naturally occurring electrical currents draw the corrosion to the anodes, Schaumburg explained.

Officials believe that an electric current, possibly caused by an electric line dislodged by the storm, may have caused the rapid erosion, he said.

The series of inspections were conducted at the Bollinger Texas City LP ship yard. Enough repair was done to allow the Elissa to sail back to Galveston, where it will remain until it celebrates the 30th anniversary of its reconstruction at a Greek shipyard.

By then the museum hopes to have raised $3 million to replace the hull as well as do a long overdue replacement of the fir deck and deck furniture, such as the companionway and skylight.

Schaumburg said officials won’t know until refitting begins whether the entire hull below water will need replacement or only the 54 corroded steel plates, each 4 feet by 10-12 feet. If all goes according to plan, the Elissa will be sailing again in 2012, he said.

The museum is negotiating with a professional fundraiser and has established a system that allows $10 donations to be made by texting 50555.

Green, a 7-year volunteer, said it is vital that the Elissa keep sailing.

“The Elissa is indicative of the types of ships that brought commerce to Galveston and to Texas,” Green said. “I think it’s important to keep that part of history for everyone to see it.”

SOURCE: http://www.chron.com/disp/story.mpl/ap/tx/7650327.html