Under attack

24 Feb 2016
High pressure: port structures need serious surface preparation. Credit: Flexcrete

High pressure: port structures need serious surface preparation. Credit: Flexcrete

Corrosion has the power to slash the life span of port structures, explains John Bensalhia

Strength has always been a common boon - think Samson's God-given powers and Superman's ability to ferry buses and trains through the skies with one finger. But even the mighty have their weaknesses: a haircut could turn Samson into a quivering jelly; a handful of Kryptonite could bring Superman to his knees. In the case of port structures, there's one crucial threat that no operator can ignore: corrosion.

A chief problem with corrosion is that all sorts of environmental nasties are lining up to attack port structures. “The highly corrosive environment is the biggest threat to port structures,” says Andrew Arnold, sales and marketing Manager, BAC Group. “Chlorides from salt, high temperatures and humidity are factors in causing steel to corrode quickly unless a suitable corrosion control strategy is adopted. The biggest concerns are when corrosion poses a risk: anything that impacts upon the safety and integrity of the structure that can pose a threat to people and the surrounding environment.”

Chris Lloyd, director, Flexcrete, explains: “Port and marine structures are typically comprised of steel or reinforced concrete. Due to the hostile environment, there are many causes of deterioration such as the damaging effects of chlorides in seawater, the aggressive action of waves and currents, plus carbonation attack. All can drastically reduce the design life of coastal defences, wharfs and jetties, leading to expensive maintenance bills and at worst, force premature demolition.”

Steel is a popular material in port construction. It's durable and cost-effective, but it's prone to corrosion in various ways, including galvanic cell formation, wave action, the impact from floating solids, and ALWC. ALWC (Accelerated Low Water Corrosion) is a particularly aggressive form of corrosion, occurring just below water level. As a form of biocorrosion, it can cause premature perforation of steel sheet piling with the potential to cause structural damage of marine infrastructure, including in ports.

Acotec explains that ALWC is a form of corrosion encountered a lot in ports and terminals and one that is not well understood as it is a combination of two research fields: biology and materials engineering. Furthermore, it causes steel to corrode at a rate of more than one mm/year, causing structures to fail prematurely, which is often not taken into account in the original design.

ALWC is a concern for many port authorities and engineers as it poses unexpected engineering and financial challenges. “Cases of ALWC have been found on structures within just 20 years of operation, requiring extensive and costly repair work on structures which would have otherwise been expected to have an effective life of 60-120 years," says Mr Lloyd. "The problem is not just associated with salt water, it has also been found on structures exposed to fresh water.

The most economical solution is to provide protection from accelerated corrosion at an early stage, thus maximising the design life and avoiding the risk of catastrophic, sudden failure with all the associated expense of repairs, lost business and risks to health and safety.”  

Bring in reinforcements

Reinforced concrete structures offer an extra level of protection as the steel reinforcement is protected against corrosion by the inherent highly alkaline environment of the concrete - created by the release of calcium hydroxide from the cement hydration.

But port operators must ensure that the level of concrete cover is adequate enough. “When concrete structures are repeatedly exposed to salt spray or submerged in saltwater, chloride ions (due to their minute size) penetrate the pores of the concrete, eventually reaching the steel, breaking down this layer and causing corrosion,” says Mr Lloyd. “Corrosion most rapidly occurs in the splash zone where the intermittently wet and dry conditions exacerbate the penetration of chlorides and there is enough oxygen to facilitate the corrosion process.

“There is also sufficient moisture present to increase the electrical conductivity of the concrete, leading to an aggressive form of localised corrosion called pitting corrosion; this can potentially cause rapid loss of steel section and major cracking and spalling of concrete, thereby compromising structural integrity.”

As soon as low concrete cover has been identified, swift action must be taken, otherwise a lack of protection for the re-bars will lead to premature de-passivation of the steel and subsequent corrosion. “Inadequate concrete cover will not only speed up the damaging effects of carbonation but also allow even more rapid ingress of chlorides, moisture and oxygen.”

The costs for corrosion that is not properly treated can be phenomenal: “I'd equate the problem with going to the dentist. If you have regular check-ups every six months there are generally less problems, but if you leave a 10-year gap between visits, then the costs for treatment are going to be more expensive coupled with suffering the pain of tooth ache!" explains Mr Arnold. 

“The same philosophy applies to corrosion control. For the structures, good engineering, materials selection and construction practises, using the right products will ensure that there is greater protection against corrosion in the long run. An effective coating and cathodic protection system are essential parts of this strategy. Regular checks and maintenance of these systems will save money and pain in the long run.”

Cost-benefits

Operators must also weigh up the cost-effectiveness of various protection methods. “When looking at the cost of providing an anti-corrosion system, it is important to consider the total cost of the application based on the whole life cycle of the structure,” says Mr Lloyd. “Cementitious coatings can offer considerable commercial advantages, both during application and throughout the design life. Application costs are considerably lower than with traditional coatings." Acotec's Humidur claims a lifetime of up to 30 years and maintenance-free coatings, with application and labour costs that are also reduced as a result of a the one-coat system.

To treat the corrosion threat to reinforced concrete, Mr Lloyd says that one option is partial recasting with new concrete. “This involves removal of the concrete back to behind the level of reinforcement typically using ultra high pressure water blasting techniques, repositioning the formwork to achieve the desired cover and recasting the concrete. However, in port and marine environments it can often be difficult to access the area to carry out remedial work.”

Testing into future anti-corrosion products continues. BAC Group has developed new technologies that utilise remote monitoring and control systems to enable delivery of accurate and online data, allowing ports to spot corrosion risks early and effectively plan maintenance and repair spending at targeted areas of their asset infrastructure.

Recently, much of the work carried out to improve the use of traditional anti-corrosion products has revolved around the use of nano-particles to improve performance. However, there are now long-term health concerns associated with some of the materials being used.

Self-healing coatings still remain the Holy Grail, says Mr Lloyd, although it can be argued that minor cracks or defects in cementitious coatings will "autogenously heal because of the potential for the cement to continue to react in wet environments". However, this is yet to be quantified.



FINDING THE RIGHT CORROSION COMBATANT

In the 1980s, the Flemish government sought a solution to the aggressive Accelerated Low Water Corrosion on local steel piles with threats to structural soundness of quay walls and other structures. Acotec can up with a solution in the form of a its solvent-free modified polyamine epoxy system, Humidur.

Humidur forms a protective barrier between corrosive elements and steel, protecting the metal substrate from corrosion by shielding it from the environment.

Acotec also invented the patented DZI cofferdam (Dry Setting Installation) to provide dry, safe access to submerged structures like sheet piled quay walls and tubular piles. It ensures the rehabilitation of quay walls and other structures in-situ without disrupting port activities.

BAC designs and manufactures cathodic protection systems that prevent corrosion on buried or submerged metallic structures.

There are two main methods of application. The first is galvanic protection: “You attach pieces of zinc, aluminium or magnesium at strategic places and in sufficient quantities to the structure you wish to protect,” says BAC's Andrew Arnold. “These anodes will then corrode away: or sacrifice themselves to protect the structure.” The second method, impressed current cathodic protection, uses a DC current which is passed through an inert anode substance onto the surface of the structure, preventing corrosion.

Flexcrete's products include water-borne, epoxy and cementitious modified polymer coating, Cemprotec E942, which is a stand-alone anti-corrosion coating for steel structures or for waterproofing/protection of concrete. The company's Chris Lloyd describes it as water-based and non-toxic, meaning that it releases no strong odours or hazardous solvents during application.

A second Flexcrete product, the water-borne, cementitious modified polymer coating, Cementitious Coating 851's, resists water to 10 bar positive and negative water pressure under a 100 metre head. A 2mm application of Cementitious Coating 851 provides the equivalent of 100mm of good quality concrete.