Zinc coatings, concrete corrosion

Table 4.81 Average corrosion rate of zinc coating inside concrete in marine environmentsa...

The external layer of pure zinc is of primary importance with regard to the corrosion resistance of the bars. If galvanized steel is exposed to a neutral environment, such as the atmosphere, the duration of protection is primarily dependent on the thickness of the zinc coating, and its composition and microstructure has a negligible effect. Similarly, for galvanized-steel bars embedded in concrete, the protective properties of zinc coatings are due for the most part to the external layer of pure zinc, which can form a passive film if it has a sufficient thickness [36-39]. In fact, a loss of thickness of 5-10 pm is required prior to passivation, while, if the thickness is insufficient, the underlying layers of Zn-Fe alloy passivate with more difficulty. 

The passive film of galvanized rebars is stable even in mildly acidic environment, so that the zinc coating remains passive even when the concrete is carbonated. The corrosion rate of galvanized steel in carbonated concrete is approximately 0.5-0.8 pm/y, therefore a typical 80 pm galvanized coating would be expected to last over 100 y. [44]. The corrosion rate of galvanized bars remains negligible in carbonated concrete even if a low content of chloride is present. 

T. G. Vinka, Corrosion of zinc coated steel in chromate reduced concrete with and without chlorides , Final Report, European Community, COST 521 Workshop, 1ST - Luxembourg, 18-19 February 2002, 59-64. 

Corrosion tests show that zinc-coated steel has a higher tolerance to moderate chloride levels in concrete than black steel (Yeomans, 1991). 

The corrosion behavior of brass and of zinc in dry cell batteries is excluded. The corrosion resistance of zinc dust paints is a special situation and is referred to only briefly. The corrosion resistance of zinc-coated steel in concrete is only summarized, as it is adequately covered in my Zinc Handbook Properties, Processing, and Use in Design (Marcel Dekker, Inc.) and in the references cited in the text. 

Galvanized reinforcement, i.e. zinc coatings formed by dipping clean rebars in a bath of molten zinc, can protect steel in concrete from corrosion attack. However, the performance reported in the literature is contradictory (Bentur et al., 1997). Galvanized rebars remain passive in carbonated concrete and the corrosion rate is much lower than with black steel. In situations where chloride induced corrosion prevails, a delay in the initiation of corrosion can be expected, but at high chloride concentrations depassivation cannot be avoided completely. 

Nickel, cadmium and zinc have all been shown to be capable of delapng and, in some cases, preventing the corrosion of reinforcing steel in concrete, but only zinc-coated (galvanized) bars are commonly available. Field studies of galvanized bars in service for many years in either a marine environment or exposed to deicing salts have failed to show any deficiencies in the concrete. ... 

Cathodic protection cannot work with prestressed concrete structures that have electrically insulated, coated pipes. There is positive experience in the case of a direct connection without coated pipes this is protection of buried prestressed concrete pipelines by zinc anodes [38], Stability against H-induced stress corrosion in high-strength steels with impressed current has to be tested (see Section 2.3.4). 

The zinc-hydrogen anode system uses 10-20 mm thick zinc sheet anodes attached to the concrete with ionically conductive hydrogel adhesive. Field trials have shown that this system is capable of supplying sufficient current for effective corrosion control. The thermal-sprayed alloy anode system utilizes a metallization (flame or arc spraying) process to form a metallized coating on the concrete surface. The two most promising anode materials were Al-Zn-In alloy and zinc (16). 

There are particular problems when the reinforcing steel is not of the conventional type of steel bars embedded directly into the concrete. There are problems for galvanized steel bars with respect to reference electrode and corrosion rate measurement because the zinc affects the readings in poorly understood ways. However, when the bar is coated in epoxy or the reinforcement is in the form of wires in ducts the problems are multiplied as described later. 

Since the maximum voltage that can be generated with zinc anodes is extremely unlikely to generate hydrogen embritdement, galvanic systems have been used to protect prestressed concrete members. They are also used on fusion bonded epoxy coated steel reinforced piles as the effects of electrical discontinuity between bars is unlikely to lead to significant stray current induced corrosion as the currents and potentials are low. 

As an example, reinforced concrete slabs are prepared for an experiment to compare effectiveness of arc-sprayed zinc and titanium anodes. All the slabs will be put under impressed current cathodic protection, and exposed to controlled applications of chloride to the concrete to promote corrosion of the reinforcing bar. Slabs are assigned randomly to be coated with either arc-sprayed zinc or titanium. In this case, randomization is a desirable precaution to control small inconsistencies due to mixing of the concrete, the chemical composition of the concrete, or the distribution of aggregate in the concrete. 

Before any repair or replacement of concrete sections is attempted, steel that is exposed by spalled or disintegrated concrete must be treated to prevent further corrosion, which may have been the initial cause of the disintegration. Rusty steel is best cleaned by sandblasting— a process that will also prepare the concrete surface by removing disintegrated and loose material. The steel should be treated with a rust-inhibitive chemical or coating, which may be a zinc-rich primer or another quality metal primer, and allowed to dry before new concrete is placed. 

In an alkali environment, the ester links in an alkyd break down and reform into alcohol and acid, (see 2.2.4.2). The known propensity of alkyd coatings to saponily makes them unsuitable for use in alkaline environments or over alkaline surfaces. Concrete, for example, is initially highly alkaline, whereas certain metals, such as zinc, become alkaline over time due to their corrosion products. 

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