Galvanic Anodes for Cathodic Prevention and Cathodic Protection Presenter Name Internal galvanic lump anodes have been around for about 15 years for use in cathodic prevention. They have proven successful in this application where the polarisation to achieve protection is low. More recently manufacturers of the original internal galvanic lump anodes have endeavoured to improve the performance of the anodes for use in cathodic protection where higher polarisation is required but that may not be possible using the lithium hydroxide based zinc activation system. In this presentation the principles of cathodic prevention and cathodic protection are reviewed to show how sacrificial zinc anode systems have been developed around a slightly acid chloride based zinc activation gel. The anode systems include: Zinc Layer Anode (ZLA) is a surface zinc foil with self adhesive backing that incorporates the Zinc Activation Paste (ZAP) Roll anodes comprising a 25mm diameter roll of the ZLA for insertion in drilled holes up to 1m long and backfilled with additional ZAP. GSC Super Anodes that are zinc lump anodes coated with ZAP and encased in a woven protective fabric.
Rebar Electrochemical Behaviour +400 Pitting Condition conducive to stable propagation of pits A D B C E Imperfect Passivity Potential (mV vs SCE) -400 Allows pre-existing pits may propagate but new pits unlikely F Perfect Passivity Initiation and propagation of pits inhibited -800 Hydrogen Discharge Oxide unstable, hydrogen formed cathodically -1200 There is a fundamental difference between cathodic protection and cathodic prevention that is critical to whether galvanic anodes work or not. A fundamental aspect is that in the area of imperfect passivity in a chloride vs potential diagram steel will continue to corrode if it has been in the pitting zone but will not corrode if it has only previously been in the area of perfect passivity. This means that the chloride threshold increases dramatically if a small amount of polarisation is applied (i.e. ABC). However if reinforcement has started to corrode due to chloride ingress (i.e. AD). and there is no polarisation then corrosion protection is low if there is only a small polarisation (i.e. DE). To protect a corroding bar a much higher polarisation is required (i.e. DF). This is of particular interest in concrete repair where cracks and spall occur at the most anodic part of a bar as chlorides ingress and the bar goes from AD. There is an accompanying potential shift around this anode as the reinforcement is polarised but even though there are chlorides around there is no corrosion of the area around the primary anode (i.e. ABC). If the primary anode area is patched then the polarisation and hence cathodic prevention of the surrounding area is eliminated and the surrounding area corrodes. This is termed the halo effect or incipient anode. If however galvanic lump anodes are embedded in the repair their lower potential polarises the surrounding rebar and this replaces the polarising effect of the primary anode and the surrounding rebar is maintained in a cathodically protected state. The key point is that the galvanic anodes only need to provide a small polarisation and as it is only protecting the immediate area around the repair the corrosion current is low and small anodes can provide a significant life. If systems used to protect against incipient anodes are only able to provide a low polarisation due to the anode:cathode area ration and nature of the zinc activation system they will not be suitable for cathodic prevention. 0.5 1.0 1.5 2.0 Chloride Content (wt % cement) AB = Cathodic Prevention with low polarisation B C= Cathodic Prevention maintained as steel does not become active A D= Corrosion initiation due to chloride ingress D E= Corrosion continues with low polarisation E F= Additional polarisation for Cathodic Protection
Anodic and Cathodic Polarisation Fe2+Fe ZnZn2+ Cathode: Anode Area FeFe2+ Zn2+Zn Log I/A When a galvanic anode is connected to reinforcement both the anode and cathode are polarised. If the anode area is small and the cathode area is high then the galvanic anode polarisation will be high and the cathode polarisation will be small. The higher the anode : cathode area ratio the greater the polarisation of the cathode. This is why some galvanic lump anode producers shape the anodes to increase the surface area. It is a way of overcoming, at least in part, issues with polarising potential of the anode itself. -0.79V -0.35V Potential E
Cyclic Voltametric Polarisation Curves a) Test Set Up c) Selected Systems 0 to -900mv ZAP (slightly acid) ZAP (alkaline) 100 Steel 10 Cathode Current Density [mA/100cm2] b)Polarisation Curves Various Systems 1 Zinc in LiOH based mortar Cyclic voltametric polarisation is a standard way of assessing the polarising potential of an anode/activation paste system. Figure a shows the test set up where the current density is measured as the a potential is applied to a circuit. In developing a Zinc Anode Paste (ZAP) the cyclic voltamteric polarisation was undertaken on a wide range of samples including the common lithium hydroxide type anodes (figure b). An enlargement of the polarisation curves is shown in figure c for two types of ZAP and a lithium hydroxide system. The intersection of the anode and cathode polarisation curves shows the expected polarisation for the anode system. As can be seen the polarising potential of the lithium hydroxide system is quite low. The ZAP paste polarising potential was increased significantly by reducing the pH to slightly acid from slightly alkaline. Hence the systems developed standardised on the slightly acidic system. The pH is just under 6 and at this pH damage to the concrete is not expected. 0.1 Expected Polarisation 0.01 -800 -600 -400 -200 Polarisation [mV Ag/AgCl]
ZAP Electrolytic Paste and Gel Auto-moistening formulation, slightly acidic but inert to concrete. Thanks to its slightly acidic formulation : zinc will form zinc ions : Zn Zn2+ + 2e Due to the electric field will attract chloride ions Final reaction Zn2+ + Cl- ZnCl2 ZnCl2 is highly soluble and hygroscopic and will therefore inhibit zinc-passivation, which occurs in alkaline based electrolytes due to the formation of insoluble Zn(OH)2 . The nature of the ZAP is critical not only from an electrochemical perspective. It has to absorb corrosion products from the anode without unduly increasing the polarisation resistance of the anode. This is not just physical as the nature of the corrosion products depends on the zinc activator as well. In alkaline systems a zinc hydroxide is formed and this tends to act as an insulator. The ZAP paste is designed (chemically and electrochemically) to produce Zinc chloride and the Zap is designed to absorb this and use it to maintain the system. maintain resistivity. This is achieved by the electrochemical process described.
Zinc / ion-conductive adhesive adhesive gel Appr. 900 micron Zinc 99,9% 250 micron 75 micron PET topliner The first of the new anode systems discussed is the Zinc Layer Anode. This anode systems comes as rolls. These are unrolled, the backing sheet partly pulled off, leaving the adhesive exposed, and the start of the sheet is pressed onto the surface to be protected. The backing sheet is then progressively removed as the sheet is pressed up to the surface. A connection is made between the reinforcement and the zinc sheet and cathodic protection is afforded to all the underlying steel. Weight : 1,750 kg/m2 Width roll : 25cm Length roll : 25m Surface area per roll : 6,25 m2
ZLA Galvanic Anodes Advantages As with other galvanic systems no wiring installation or external powersource needed Simplest of any galvanic CP system to apply Applied to surface so Doesnt need coring to insert anodes Prevents further penetration of contaminants Provides high cathodic polarisation due to the design of the ZAP The very high anode : cathode area ratio The high polarisation leads to Good throw. Protects both rebar faces in thin elements Ensures EN 12696 cathodic protection criteria are met Full design possible as anode area know Over 10 years experience
ZLA Galvanic Anodes Precautions Clean, coating free concrete surface required Avoid polymere based repair-mortars Design system for each situation Design life based on chloride penetration and anode:cathode ratio (typically 20-40years) SRCP galvanic anode design spreadsheet freely available Waterproof system to protect ZAP Overcoated with approved coating where water exposed Seal exposed edges with approved sealant Seal leaking cracks through concrete Include coating resitance layer to extend life (>40 years) where large throw and low polarisation required
ZLA to Gouda Bridge Soffit, Netherland Repair at column support Connection of anode to rebar
Balconies at Dordrecht, Netherlands The structure before repair Slab after removal of drummy concrete & establishing rebar continuity Slab after pouring repair with rebar connection showing The repaired balcony with ZLA applied ZLA after covering entire surface with 2 layers of Mapelastic Smart by roller Decorative finish after application of 2 layers of Mapei Elastocolour Pitura
ZLA To Soffit of an Apartment Balcony Typical damage Patch Repair ZLA Application Finished Job
ZLA Protection of a Beam
ZLA Application to Bridge Application of ZLA to a bridge beam and column Finished ZLA application before coatings
ZLA Application to Bridge Columns Starting ZLA application After application of all ZLA During application of base coat After application of decorative coating
ZLA Rebar Connections & Fixings Mechanical Fixings (if Required) Sheet fixed to concrete using adhesive backing Holes drilled through sheet & concrete Plastic nails hammered through ZLA into concrete where required. NB: the adhesive alone provides adequate fixity for applications where the consequence of a sheet falling off is low. On road bridges the consequence could be high and physical attachments is appropriate. Rebar Connection (option) Sheet fixed to concrete using adhesive backing Holes drilled through sheet, concrete and rebar Metal nails hammered into steel at appropriate centres NB : Preference is to use full rebar connections Concrete metal nails
Polarisation Checking Circuits Typically galvanic CP systems do not have checks of the polarisation to European and Australian codes. With the systems described here such checks are recommended. To do this the wiring is more complex but by checking systems at a couple of locations a high degree of confidence in the system is achieved.
Roll Anode & Anode Paste Roll anode showing foil layers and connection Zap cartridges Drilling holes for Roll Anodes Filling around the Roll Anode with ZAP Roll anodes are a rolled sheet of ZLA and can be supplied in any length up to 1m long. The rolled ZLA is used so that there is a layer of ZAP between each zinc layer and this assists in ensuring the corrosion products do not have a significant effect on the anodes polarisation resistance. A 32mm hole of an appropriate depth is drilled and the roll anode is inserted. The ZAP paste is then gunned in around the roll anode. The ZAP paste can also be used for other anodes.
Roll Anode as Internal Anode Corroding Bar Patch where concrete is damaged. Clean rebar to bright steel in patch area. Prepare concrete surfaces in accordance with patch materials manufacturers recommended procedures Electrical connection between Roll Anode and rebar Roll Anode in electrolyte gel Chloride contaminated concrete Corrosion protection of nearside bars Corrosion protection of far side bars depends on proximity of roll anode Roll anodes can be used as internal anodes to provide local protection. This is particularly useful where there is deep chloride penetration such that the rear of the second layer of steel is corroding. Only spalls need to be patched as the roll anode can be used to protect all rebar around and below the patch whereas anodes just inserted in the patch may have a limited depth of influence. Existing ties provide electrical continuity
Roll Anode As Internal Anode Chain of anode connections Connection to rebar Zinc Roll Anode Roll anodes are generally arranged in a chain systems when used for general protection of larger areas. Where performance checking is required the chain and half cells are wired back to the corrosion monitoring station. 1m long roll anodes have been used to provide protection to the rear of bridge abutments in the Netherlands. Electrolyte gel Column reinforcing grid (links not shown for clarity)
ZLA to Gouda Bridge Beams
GSC Super Anodes GSC Super Anodes Packing Storage GSC 10/10 – 100mmx55mmx12mm GSC 10/20 – 100mmx55mmx15mm GSC 30/10 – 300mmx50mmx10mm GSC 30/12 – 300mmx50mmx12mm Packing GSC 30/10 & 30/20 12pcs/Carton GSC 10/10 & 10/20 24pcs/Carton Storage Shelf Life (nominal): 12months. Preferably temperature <30°C & RH<50% GSC 10/10 GSC 30/12
GSC Super Anodes in New Structures Lightly reinforced columns Floor Slabs and Beams Heavily Reinforced Columns Bridge deck or viaduct beam supports and columns Bridge decks Zones of newly casted concrete adhered onto an existing structure Balcony facings and concrete facades Floorings GSC Super anodes are mainly used in new structures for cathodic prevention. They are used wherever there is an unacceptable risk of corrosion and can be an economicl means of reducing cover required.
GSC Super Anode Simple installation, needs no anode wiring. Hence low cost Pre-stressed or post-tensioned tendons will not be subject to hydrogen embrit-tlement due to reasonable operating potentials Need no service or monitoring Cathodic Protection When proper designed will give enough current in dry and arid environments for 20 year life When applied in aggressive environments the current densities are highly compatible with impressed current systems. Cathodic Prevention Extremely adapted for patch repairs Current densities are natural self-adjusting depending on the current request by the structure. Low potential shift gives high protection but current density is low and anode life is long
GSC Super Anode Installation Remove cracked and delaminated concrete Tie on anodes and check electrical continuity. Resistance between anode and rebar should be less than 1 ohm. At normal steel densities and for a design life of around 15-20 years anode usage is: GSC 30/10 & 30/20 anode 3pcs/m2 concrete GSC 10/10 & 10/20 anode 4pcs/m2 concrete Reinstate with conventional patch materials with a resistivity as low as practically possible.
Galvanic Anode Data Logger Turn knob to rotate through display options LED indicator LCD display (2 line display with 8 characters per line) for : Date Time Channel 1 (mv) Channel 2 (mv) Channel 3 (mv) Channel 4 (mv) SD memory card for data storage Start Depolarisation switch
Half Cells for Potential Measurement ERE 20 Manganese Dioxide Electrode 30 years of experience Stable and robust The ERE 20 is a true, long life Reference Electrode, which can be cast into the cover concrete to check the cathodic protection and to monitor the corrosion state of reinforcing steel or predict corrosion. Normally in newly cast concrete structures, but the electrode can also be installed in existing structures. Based on proven battery technology, the ERE 20 is a true half-cell using a manganese dioxide electrode in steel housing with an alkaline, chloride-free gel. The steel housing is made of a corrosion resistant mate-rial. The pH of the gel corresponds to that of pore water in normal concrete, so errors due to diffusion of ions through the porous plug are eliminated. The potential of ERE 20 is virtually independent of changes in the chemical properties of the concrete. It can, therefore, be used in wet or dry concrete, whether exposed to chlorides or to carbonation.