Typical Costs for Extra steel Required in Sacrificial Thickness

The pricing in the table below is an approximate price of the additional steel required in sacrificial thickness, and it is based on steel price for structural sections. This is excluding additional costs and is based purely on steel price, while also assuming the minimum sacrificial thickness allowable.

The protection offered by Cathodic Protection (CP), design life of 30 years, usually is significantly less expensive than the sacrificial thickness.

The design for corrosion protection is dependent on exposure area, as with its increase the mass of steel loss increases. By increasing the sacrificial thickness, the total mass of steel increases, whilst not guaranteeing the design life. The increase of exposure area also requires an increase in cost with CP, the reason for this is the fact that the anode mass is dependent on the area. Typically for an element without paint cover it requires an estimated (at current prices) 90 AED/m2, and for an element with paint cover it reduces to 55 AED/m2, in accordance with the DNV standard. Cathodic protection is a proven technology and the likelihood of corrosion with is significantly less.

There is a misconception of the maintenance cost of cathodic protection. Typically, once installed, these systems self regulate and require little or no on-going costs. International standards do not have a scheduled requirement and many times it is the owner’s apprehensiveness about the system that leads to excessive looking after. For example, it is possible to install a CP system on a jetty using sacrificial alloy anodes and not need to inspect for upto 3 years.

Optimisation

The severity of corrosion for a steel member in a marine environment varies depending on the location relative to sea water. Most design codes specify this and advise that the design can be optimised based on these corrosion rates.

Figure 1: Corrosion Rate Distribution

Figure 2: Relative Loss in Metal Thickness

The section just below MLW experiences some of the highest corrosion, and this section is most prone to ALWC attack. This is an area that can be actively protected by a CP system, thus inhibiting and limiting corrosion. Therefore, negating the need of excessive sacrificial thickness for protection and insuring that the structure does not deteriorate before the allotted time.

Conclusion

The primary reasons for corrosion protection safety for structure against failure, to prolong the life span of the element, and to reduce the total project and operation life cost. Unforeseen failure to structural elements is both dangerous and very expensive, as remedial action is far more difficult manage in comparison to providing a protective system from the beginning. A sacrificial thickness is a good methodology to obtain durability with regards to conventional, uniform corrosion. However, in conditions of extreme localized corrosion attacks it can only limit the ingress for a short period of time. For protection against these, one of two methods are recommended. First, is regular monthly inspection of all elements at risk, so that remedial action can be taken in the early stages before the reduction of safety factors. Secondly is the installation of a cathodic protection system, which is also recommended by CIRI C634, which is proven to provide protection against all forms of electro chemical corrosion, this system requires annual monitoring only.

Corrosion due to Accelerated Low Water Corrosion (ALWC)

Accelerated Low Water Corrosion (ALWC) is a relatively new phenomenon, and an extreme form of aggressive corrosion, that majority of the time occurs slightly above Lowest Astronomical Tide (LAT) level, and is reported to have occurred along submerged sections. The occurrence is on unprotected steel in tidal areas. The cause of this is due to bacterial activity, and is therefore a microbial influenced corrosion (MIC). This occurs when sulphate resisting bacteria, an anaerobic bacteria, grow on steel forming a colony, if growth is sustained for long enough it forms a biofilm. This patch of bacteria does not directly consume the steel; however, it promotes and aggressively increases the rate of corrosion as it makes the ideal environment for it.

According to CIRIA C634 this process is random, and a successful method for predicting its occurrence has not been developed. Cases of ALWC have been reported from around the world in all tidal areas, and cause of its occurrence has not been truly understood. Its high variability is baffling as variation occurs in the local geography, where some piles are found with ALWC and some piles within the same vicinity are found to be free of it. The time scale is variable also as it is a multi-stage process and not linear like in table 2, which underestimates ALWC, as the rate of corrosion varies depending on the micro-environment. However, once the biofilm has formed rates of metal wastage is very high, making it possible to see patches within a couple of years. As a rule of thumb localised corrosion rates are 1.5 to 3 times more than the general uniform corrosion rates.

Currently the only reliable method of detecting ALWC is by visual inspection together with residual wall thickness measurements. ALWC occurs as localised patches of damage, identified by a characteristic, poorly-adherent orange corrosion product over a 'soupy' black underlayer associated with rapid metal thinning. 

The strategy for management of ALWC will depend on whether the structure is new built or an existing structure. The corrosion protection measures that are currently applicable to ALWC are those based on conventional corrosion control methods such as cathodic protection (CP) and coatings of various types. 

Conventional Steel Corrosion and Durability Design

Conventional corrosion is an electrochemical redox reaction, thus when steel is in contact with an electrolyte and oxygen, then steel mass will be lost, this is more pronounce in sea water. Corrosion, compared to time is generally a linear process and is uniformly spread over the exposed area.

Table 1. Recommended value for the loss of thickness (mm) due to corrosion for piles and sheet piles in fresh water or in sea water

On the basis of this table the common method utilised in accounting for corrosion is to utilise a sacrificial thickness by increasing the thickness of the pile by at least 4mm.

However, for construction in the Arabian gulf this method may not be the optimal solution due to the climatic and seawater conditions. The gulf coastline experiences some of the most extreme weather conditions with summer temperature reaching up to mid to high forties, with the salinity of the Gulf generally being highly variable with some sections near the coast reaching a concentration of 10 % (Fookes et al). In general, the salinity of the Gulf, at 4 %, is also higher than the open ocean, at 3 %.

The sacrificial thickness specification for a pile in sea water in zone of high attack is 3.75 mm, which means that a corrosion rate of 0.075 mm/year is adopted. However, according to research presented in CIRIA C634 that is the minimum rate of corrosion reported. The average corrosion rates reported range from 0.08 to 0.2 mm/side/year. For the harsh aggressive environment of the Arabian Gulf compounded with high and variable salinity of sea water, with the high temperatures a higher corrosion rate in design is recommended for optimal durability. The highest corrosion rates range from 0.17 to 0.34 mm/side/year. For a worst-case scenario, the highest corrosion rate will see a loss of 17 mm of steel, and if a sacrificial thickness of 4 mm is utilised, it will only protect the integrity of the member for 12 years.

Table 2. Corrosion Rates found in Literature

Biodiesel Bacteria Boost Corrosion of Underground Storage

Thousands of underground diesel storage tanks may need to be replaced in the coming years because bacteria present in biodiesel are boosting corrosion, the Volkskrant has reported.

The paper bases its claim on a preliminary report by SIKB, which monitors ground pollution in relation to infrastructure. It has been compulsory to add biodiesel – fuel produced using plant or animal fats – to traditional diesel since 2007 and since then there has been an increase in corrosion reported in uncoated steel tanks, the institute says in a new report.

The decision to remove sulphur from diesel, which slowed down bacterial growth, has had an additional impact.

Although the SIKB research focused on underground tanks without protective coatings, there are also strong indications that other sorts of tanks are also affected, the report said.

The transport ministry said in a reaction it would wait for a definitive report before deciding what action to take.
 

Concrete Cancer

The Middle East is well known for the presence of a very aggressive salty water table that sits barely a few meters below the surface. As we all know, salt and water coupled with heat are the perfect blend to create corrosion nightmare of concrete structures.

Some Facts

Concrete Cancer, often identified by flaking concrete or rust stains, which originate deep within the concrete is a serious problem caused by corroding/rusted reinforcing steel from within the concrete. As steel rusts it can expand up to 7 times its original size causing the surrounding concrete to crack. As the steel pushes the concrete away, more water gets to the steel expediting the process.

The process is generally due to:

·       Presence of large quantities of water and salt

·       The ends of reinforcing being too close to the surface allowing water to seep through concrete and react with the steel

·       Poorly treated reinforcing steel being used in the original pour of the slab

·       Fractures in the concrete allowing water to penetrate the concrete and react with the steel

What do we do?

Spalled concrete can be a safety hazard. Concrete cancer and delaminated concrete should be treated immediately as deferring the treatment will inevitably lead to increased problems into the future.

Similarly, treating the visual aspects such as rendering over the steel are short-term solutions as the rusting process will continue below the surface causing the steel to again displace the concrete and in some cases, rust so badly the steel eventually needs replacement. This approach – we call it the ‘make up’ approach – is aesthetic. In essence, the ugly bits are removed and given a nice clean looking finish, however the underlying problem is very much still present. Within a short time, the area adjacent to the area repaired is cracking and breaking and requires repair. You are back to square one.

The Real Stuff…

The appropriate and effective treatment necessary is cathodic protection – an electrochemical method of arresting corrosion for an extended period of time – ranging from 5 years to 50 years.

Ducorr’s SHIELD™ technology is easy to install into dilapidated atmospherically exposed concrete areas and achieve excellent corrosion protection. The system uses permanent power to provide sustained protection by simply making the corrosion reaction impossible to occur. There’s lots of thermodynamic theory behind, which would be too long for this article – but in essence cathodic protection is the ONLY method that address corrosion at an elemental level eliminating the possibility of any further damage.

The Dubai Water Canal is key infrastructure project that involves the construction of water canal that routes just east of Sheikh Zayed Road to the Jumeirah beach. The canal mainly consists of block wall construction. However, in a minor section of the canal, the construction incorporates a reinforced concrete diaphragm wall. The project specification requires that the reinforcing steel of this diaphragm wall be protected from corrosion using cathodic protection designed and installed by DUCORR.

Contact us to deploy your system now.