Seawater galvanic coupling

In galvanic coupling, titanium is usually the cathode metal and consequently not attacked. The galvanic potential in flowing seawater in relation to other metals is shown in Table 10. Because titanium is a cathode metal, hydrogen absorption may be of concern, as it occurs with titanium complexed to iron (38). 

Structures immersed in seawater. Macrocells may form between rebars reached by chlorides and passive rebars on which, for any reason, oxygen is available. Macrocell current is then controlled by the amount of oxygen that can be reduced on the passive rebars. The galvanic coupling lowers the potential on these rebars and produces alkalinity on their surface. Therefore the macrocell contributes to maintaining the steel passive. 

Rebars not entirely embedded in concrete. Macrocell corrosion can occur when there are macroscopic defects in the concrete (cracks with large width, honeycombs, delaminations, etc.) or when there are metallic parts connected to the rebars that are only partially embedded in the concrete. This case is important for structures immersed in seawater or in aggressive soil. Besides being subjected to direct attack, those parts in direct contact with water or soil may also undergo more severe attack caused by the galvanic coupling with steel embedded in concrete. 

Aluminium in contact with structural steel. Although steel corrodes faster than aluminium in environments such as seawater, marine and industrial atmospheres, water containing SO2, and soft water, aluminium is the material that primarily corrodes when these two materials are galvanically coupled in the mentioned environments. The reason for this apparent paradox is that the corrosion potential of... 

The reservoir of carbon steel plated with stainless steel, with the latter in contact with seawater, would be quite probably the most rapidly perforated. Indeed, the localized corrosion on stainless steel, once it reaches the carbon steel, would allow the galvanic coupling between the cathodic high surface area of stainless steel and the anodic small surface area of carbon steel, exposed through the holes in the stainless steel layer, with consequent rapid perforation. 

To avoid the galvanic coupling between different materials, especially when the electrical conductivity of the solutions is high. A higher conductivity corresponds to a higher extent of the cathodic area related to the same anodic area or vice versa. Sometimes, because of differences in degradability, it may be appropriate to use bolts and/or weld material more noble in comparison to the materials that are connected, but we must keep in mind that the electrochemical scale of nobility is not always that of seawater but depends on the environment and the pollutants. Copper is normally more noble than iron, but in the presence of sulfides, the practical nobility of the two materials can be virtually coincident, while in the presence of ammonia, the nobility of copper may be lower than that of iron. Stainless steel is usually more noble than carbon steel, but in aerated alkaline enviromnents, the order of nobility can be inverted. 

W C Tucker and R Brown, Blister formation on graphite/polymer composites galvanically coupled with steel in seawater , J Compos Mater 1989 23(4) 389-395. 

The attraction of rubbed amber and some other effects of electricity were known in ancient times. We know from finding nails in an old wreck that the Romans knew about contact corrosion combined with electric current How. A skin of lead as a protection against boring worms covered the wooden planks of the ship and was nailed down with copper nails. Galvanic couples formed between the lead and die copper nails and the less noble lead she around the nails corroded in the seawater and fell off. The shipbuilders discovered a simple solution and covered the heads of the cqpper nails with lead as well. Galvanic current Dow between the two metals was eliminated and corrosion was jnevented (26). 

Thus, it is important to use specimens with large free surface area, particularly if the environment has high electric conductivity that allows long-distance galvanic coupling (seawater, for example). 

Referring to the galvanic series of some commercial metals and alloys in seawater, mark the condition which would lead to minimum corrosion by galvanic coupling. 

TABLE 8.18 Galvanic Couple Data for C70600 and C71500 with Other Materials in 0.6-m/s Flowing Seawater (One-Year Exposures -Equal Area Couples)... 

When in contact with other metals, titanium alloys are not subject to galvanic corrosion in seawater. However titanium may accelerate attack on active metals such as steel, aluminum, and copper alloys. The extent of galvanic corrosion will depend on many factors such as anode-to-cathode ratio, seawater velocity, and seawater chemistry. The most successful strategies eliminate this galvanic couple by using more resistant, compatible, and passive metals with titanium, alltitanium construction, or dielectric (insulating) joints. 

This method is widely used in ship building for the neutralisation of galvanic coupling with a sacrificial anode, generally made in zinc or in a specific aluminium alloy (Figure B.5.4). In order for this protection to be effective, all metallic parts must be at the same potential and the anodes must not be painted they should aU be inspected regularly to check whether they are in perfect working order and to replace them as necessary. In seawater, such anodes are effective up to a distance of 10 m. 

In practice, when assemblies of aluminium with other metals (including stainless steel) are permanently immersed, the contacts must be insulated in order to avoid galvanic corrosion. This precaution becomes more necessary as the conductivity of the water rises. In seawater, it is possible to neutralise galvanic coupling by using consumable anodes, if the geometry of the structure is suitable (see Section B.5.5). Consumable anodes are inefficient if the conductivity of the water is low or if the stmcture s geometry is complex, which is often the case inside the ballast tanks of ships. 

The very high electrical conductivity of seawater favours the development of galvanic corrosion, which is why galvanic coupling in heterogeneous assemblies between aluminium and other metals must be neutralised. 

Apart from the selection of the crevice former device, it is important to have in mind that the severity of the test may depend on the availability of cathodic current. Thus, it is important to use specimens witii large free surface area, particularly if the environment has high electric conductivity that allows long-distance galvanic coupling (seawater, for example). 

Cathodic protection Because crevice corrosion occurs above a critical potential, cathodic protection is an efficient way to prevent crevice corrosion by maintaining the potential of the free surfaces below the protection potential. This is the case of 17-4-PH stainless steel, which is not resistant to crevice corrosion in seawater but is commonly used because it performs very well under cathodic protection (this protection is often due to galvanic coupling to unalloyed steel). 

Cathodic protection (CP) is an electrochemical technique of corrosion control in which the potential of a metal surface is moved in a cathodic direction to reduce the thermodynamic tendency for corrosion. CP requires that the item to be protected be in contact with an electrolyte. Only those parts of the item that are electrically coupled to the anode and to which the CP current can flow are protected. Thus, the inside of a buried pipe is not capable of cathodic protection unless a suitable anode is placed inside the pipe. The electrolyte through which the CP current flows is usually seawater or soil. Fresh waters generally have inadequate conductivity (but the interiors of galvanized hot water tanks are sometimes protected by a sacrificial magnesium anode) and the conductivity... 

The galvanic series of metals and alloys in seawater is given in Table 7.20. From this series it is clear that steel and 2024 aluminum are in close proximity. From their positions it is inferred that steel is cathodic and aluminum is anodic in seawater. The corrosion potentials of iron and aluminum measured after immersion in various media for 24 h are given in Table 7.21. It is seen from these data that the corrosion potentials of iron and aluminum are very nearly the same in 0.1M sodium chloride. Some studies on the galvanic action of the steel-aluminum couple in fresh waters such as pure, river, lake and underground water and salt solutions are noted in Table 7.22. In one of the studies, the... 

Galvanic corrosion rates (mils) of some couples after 16 yr exposure to seawater and fresh water are given in Table 7.23. In the cae of carbon steel/aluminum the data show that in fresh water carbon steel corrodes to a greater extent than aluminum which provides further evidence for polarity reversal of the steel/Al couple in fresh water. 

As mentioned above, the environment has a significant effect on whether or not galvanic corrosion will be a problem. For example, carbon steel will corrode rapidly if equal or larger areas of Monel 400 are coupled with it in seawater. Conversely, carbon steel is compatible with Monel 400 in concentrated caustic solutions. Even freshwater can be sufficiently different from seawater couples incompatible in seawater work well in freshwater. For example, copper-steel and aluminum-steet couples are satisfactory for handling... 

Ship hulls Painting cannot always protect hostile marine conditions, in ships and, areas above keel blocks. Stem and mdder areas suffer erosion and corrosion due to the high turbulence caused by the propeller coupled with the galvanic effects of the noble bronze propeller. Effective cathodic protection of ship hulls and similar marine structures in seawater against corrosion can be apphed using either aluminum or zinc alloy sacrificial anodes. Twenty percent of the anodes required for full hull protection are required for stern protection only. 

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