General consideration

In today’s world, saving raw materials and energy is an absolute priority. The steadily growing demand for fuels and metals has led to an exponential increase in their price. This phenomenon is exacerbated by the impetuous increase in demand from countries with strong industrial growth, such as China and India.

The consequent logic of saving resources requires that the works carried out are effectively preserved from the risks of deterioration. This means that it is necessary to foresee and calculate the expenditure required for corrosion protection as a fundamental part of investments also in the field of construction and steel products.  From an economic point of view, the question is not of secondary importance.

Already in the 80s, a study carried out in Italy put around 40,000 billion lire at the time, an estimate of the total costs attributable to corrosive phenomena. From calculations made a few decades ago (1980s) in the United States and Europe, the annual cost of corrosion could be estimated in the order of 4% of GDP. It is not unreasonable to think that the incidence of the cost of corrosion has increased in the last period due to the effects that the economic situation of recent years has had on the cost of materials and energy, resources necessary for the partial or total renovation of the compromised works and for repair of protective systems with maintenance.

Any steel artifact must be protected from corrosion in any case. The efficiency of the protective system in economic terms is basically measured in terms of quality and durability. There is no doubt that opting for a low initial cost method, as is the case with a low-cost rust remover, but of poor effectiveness and insufficient duration, jeopardizes the integrity of the work and jeopardizes its continued usability over time. Downtime should be considered in the balance sheet since many times they constitute the worst damage to lost revenues, as is particularly evident in the case of industrial structures. In addition, the use of non-durable protection techniques, but only apparently cheaper, forces, after a certain period of time, to the considerable expense of reintegrating the protection in situ with the difficulties and the economic burden that this entails.

The discriminating factors in the choice of corrosion protection

On the other hand, a good anticorrosive treatment must enjoy a series of fundamental characteristics, which absolutely cannot be ignored in order to give the metal product the most suitable protection:

• Duration
• Reliability
• Ease of application
• Ease of inspection
• Ease of repair
• Short-term completion time.

To make a coherent choice in terms of costs, it is necessary to consider not only the initial cost of the protection but also the costs of any maintenance required during the overall life of the product. The performances obtained by the different materials are obviously fundamental discriminants to distance over time or completely avoid the aforementioned interventions with the relative costs. The protections of an organic polymeric nature in general, the paints, may sometimes have lower initial costs, but are characterized by variable reliability and require continuous checks and frequent maintenance. On the other hand, there are quality organic products on the market which, although removing the need for maintenance over time (duration of the order of a decade), involve higher initial costs than galvanizing. In fact, in recent decades there has been a notable increase in the costs of the raw materials from which the paints are prepared, largely derived from petroleum. Furthermore, their chemical composition is constantly evolving to make the products comply with the increasingly stringent legislative requirements relating to VOCs, volatile organic compounds which are constituents of solvents.

In any case, paints exhibit greater exposure to possible mechanical damage and progressive decay of properties also simply due to exposure to sunlight and changes in temperature. In a short time, they can require maintenance interventions and significant expenses that are often unexpected because they are unpredictable (as in the case of accidental impacts, abrasions and hidden application defects).

The surface preparation phases of the product constitute a fundamental step for the future performance of the protection offered even by the best of paints. Application errors in this phase can affect its duration. It should be considered that in most cases the application of the paint is carried out when the product has already been installed, therefore, without the possibility of carrying out the preparation in the context of a robust industrial-type process. This adds another determining risk factor to the anti-corrosion investment.

An alternative to protective coatings is galvanizing, whose adhesion to the surface of steel and iron is already intrinsically guaranteed by the development of the zinc layer. The duration, and therefore the value, of the galvanizing is directly proportional to the thickness detected and, in current practice, it is always or almost always that the thicknesses applied are higher than those required by the reference standards. Given the corrosion resistance offered by zinc, which we have discussed extensively in the chapters dedicated to the topic, we can be sure of the long duration of the protection, able to remain for decades in all the usual environments of use.

The cost of producing a galvanizing coating depends on the thickness of the steel of which the product to be protected is made. Consequently, the comparison with other protective systems can become complex in those cases in which the weight per beam length of the product increases. Only in some particular cases, hot dip galvanizing is initially more expensive than some painting systems.  This obviously has important repercussions for economic evaluations and suggests that the comparison is made considering the Life Cycle Costing criteria.

Initial costs discounted and final costs

There are several methods to calculate the advantages and disadvantages associated with the adoption of a long-term corrosion protection system. The most commonly used method is the calculation of the Net Present Value (NPV) of each of the systems being compared. This calculation takes into account the initial costs, retouching costs, maintenance costs, repainting costs, inflation and indirect costs, during the time spent in efficiency of the structures to be protected.

The Net Present Value follows the method adopted by companies to evaluate the value of an investment.

• NPV = NFV/(1+R)n
• where NFV = (current cost) * (1 + I) n
• NPV, net present value
• R, interest rate
• n, life time of the structure
• I, inflation rate.

In the United States, studies were conducted based on data produced by the American Association of Galvanizers and the National (American) Association of Corrosion Engineers, which made a comparison, the results of which can be summarized in Tab. 8.1.

The prices, although fairly recent, refer to a few years ago, so they are not indicative of the current market values, but are purely for illustrative  purposes. The values expressed in Tab. 8.1 enhance the effectiveness and cost-benefit ratio offered by galvanizing, which is also convenient in cases where the initial price of other methods is significantly lower; and this is caused by the fact that galvanizing does not require maintenance for decades. The result is also favored by the fact that the galvanization provides adequate coverage inside the hollow tubular profiles and in hard-to-reach locations. In addition, it ensures uniform thickness even on edges and edges resistant to abrasion.

To schematize what has been stated, let’s consider exposing a protected structure to a corrosive atmosphere of medium aggressiveness:

As stated in chapter 3, it can be assumed that the galvanizing will last in service of about 20-25 years for coatings equal to 120µm on medium carpentry (5mm thickness or 55m2 / ton). As stated in chapter 3, it can be assumed that the galvanizing will last in service of about 20-25 years for coatings equal to 120µm on medium carpentry (5mm thickness or 55m2 / ton). Note that, in this case, the galvanizing still exerts its protective capacity and the product is still perfectly intact.

In comparison with a typical painting (one coat of primer and two of topcoats of about 80µm) it must be borne in mind that the duration does not exceed 8 – 10 years before 5% of the surface becomes covered with rust and the first maintenance is required ( according to EN ISO 12944).

As a third alternative, let’s imagine applying a duplex system, for which we can assume 120µm of galvanizing layer and 160µm of painting in two coats. Bibliographic references suggest that, if quality materials are used, it can easily exceed 75 years without the need for maintenance of the duplex system.

It is noted that in the long term, even the initially more expensive system can become advantageous from an economic point of view, so the choice of the anticorrosive system cannot be separated case by case from an analysis of the discounted costs for the desired duration.

As mentioned above, it is generally not true that hot dip galvanizing has higher initial costs than painting. On this aspect, it may be important to warn users against a misleading prejudice: it is common opinion that a system with high performance must be expensive, or rather, more expensive than the others. Hot dip galvanizing, while ensuring high performance, has a more than competitive cost with quality paints, the only ones with which it makes sense to make a comparison, compared to which it is often much cheaper already at the time of its application. . The initial cost of the galvanizing treatment compared to other systems is generally lower than an organic three-coat coating carried out with manual methods (brush or spray), as often happens on construction sites, in the case of painting after installation. In these conditions, the difference is the use of labor, which has a greater impact in the case of painting.