Anti-corrosion protection of cast iron

Cast irons are alloys of iron, carbon and accompanying elements (desirable and undesirable), where the carbon content is higher than 2%, and the sum of all accompanying elements does not exceed 2%. Cast iron is a popular construction material, especially for its simplicity of production. Basic types of cast iron are distinguished according to the shape of the graphite in the alloy, and each group has its own characteristic physical-mechanical and chemical properties. Cast iron with flake graphite (grey), with nodular graphite (ductile), with worm (vermicular) graphite, white cast iron, tempered, ADI cast iron. Each type of cast iron can be further modified with alloying additives, inoculation and heat treatment. Therefore, it is not possible to establish clear general rules for the behavior of cast irons against corrosion and their resistance. However, due to the fact that the resistance to corrosion cannot be increased much by low alloying, it is true that cast irons tend to be more sensitive to corrosion than alloy steels.

Both chemical and electrochemical corrosion are used for cast iron. (Non-chemical corrosion, for example cavitation, comes into consideration only for some products.) Chemical corrosion is the action of oxidizing or reducing substances without the presence of an electrolyte and leads to the formation of a layer of waste products at the point of phase contact. Its speed increases at temperatures above 580°C. More dangerous is electrochemical corrosion, which causes the transfer of charge along the metal grid based on the action of the galvanic cell. It is created in the electrolyte not only in the presence of two different metals, but also an unevenly treated surface of one metal is enough to create it. Electrochemical corrosion is accelerated if chlorine ions (Cl ^- ), carbonates (CO ₃ ^2- ) or sulfates (SO ₄ ^2- ) are present in the water. Corrosion resistance does not depend only on the composition of cast iron, but also on the concentration of corrosive substances.

Preventing the occurrence of corrosion during the production and processing of cast iron is very problematic. Here are some general guidelines to reduce this risk:

• use such binders and form separators that do not contain oxidizing or reducing substances and these are not created even by their thermal decomposition,

• unmould the castings at temperatures below 500°C - the lower the temperature, the better,

• work with products above the dew point temperature,

• prevent contact with water,

• if the technological process requires contact of the product with water, it should not contain chlorine (free or bound), carbonates or carbonic acid, sulfates, saturated oxygen.

These conditions are almost impossible to meet (a number of aqueous degreasing liquids contain NaHCO ₃ , there is always some oxygen in the water, etc.). That is why procedures that prevent the formation of corrosion or its spread are used more in practice. They are both physical and chemical.

1. Physical ones work by creating an impermeable layer on the surface that resists the diffusion of electrolytes, oxidizing or reducing substances and has a hydrophobic character.

• plating Cr, Ni, Co, Au, Zn, etc.,

• plastic coating, most often PVC, PP, PE,

• measures with protective coating, varnish, paint,

• by applying a hydrophobizing agent, oil, wax, silicone, fluorinated hydrocarbons, amines, etc.,

2. Chemical methods work on the basis of a chemically or physically bound impermeable layer on the surface of the metal, which either transforms the metal into another corrosion-resistant compound, or prevents the transfer of the corrosion ion to the metal through a chemical redox reaction, or acts as a free radical scavenger, or acts as a cathode/ anode.

• passivation by oxidation to Fe ₃ O ₄ , blackening,

• passivation with organic salts, oxalate, citrate, tannate, chelate, etc.,

• passivation with inorganic salts, chromating, phosphating,

• inhibition, e.g. amines,

• inhibition using free radical scavengers,

• cathodization/anodization,

3. A combination of both principles.

The following table provides a basic comparison of the mentioned principles:

* If it is not a coating that also combines chemical protection

In practice, a combination of both of the above-mentioned principles is very often used. E.g. the base paint contains zinc (cathodic protection) and the top paint has hydrophobic and barrier properties. The blackened surface is treated with a preservative. The ongoing corrosion is stopped by the reaction to the organic salt and simultaneously converted to a metal polymer with barrier properties (corrosion converters). The preservative oil with hydrophobic properties contains corrosion inhibitors and free radical scavengers.

From the point of view of interoperational protection of cast iron products against corrosion (ie short-term up to 1 year), the following means and principles are most often used today:

• Painting. It works with its hydrophobic and barrier principle, but it is very problematic because the varnish must be removed before using the product or its final treatment.

• Oil preservation. Again, it is necessary to degrease the product before use (in most cases). However, this is easier than removing the paint. On the other hand, an oiled surface sticks the products together, catches dust and deteriorates the packaging.

• Inhibition using contact water-soluble corrosion inhibitors. It is a very easy application, where usually it is not necessary to remove the inhibitor micro layer before the next process or final treatment, but this micro layer is sensitive to moisture. Running water, rain or condensed moisture will wash away the inhibitor.

• Inhibition using vapor/Volatile Corrosion Inhibitors (VCI). They are similar to contact inhibitors, but they also have the property of slowly evaporating. The vapors then remain attached to the metal surface where the inhibitor did not reach. This method is used where the products are packed in a closed package.

• Inhibition of organically soluble corrosion inhibitors. These agents are easy to apply, the size of the deposit can be controlled by viscosity. After application, the solvent evaporates, leaving only a microfilm on the surface of the product, which has hydrophobic properties. A disadvantage may be the flammability of the vapors.

An example of such a product is the washing, cleaning and preservative liquid KORING 141 (for ferrous metals) or KORING 145 (for ferrous and non-ferrous metals). It is a solution of corrosion inhibitors in organic solvents. The liquid combines several processes. It degreases the product and cleans it of chips and mechanical dirt stuck to the surface. Its viscosity can be adjusted within a certain range to the customer's requirements. Thanks to the composition of corrosion inhibitors, pitting corrosion is simultaneously removed in many cases. After the solvent has dried, the surface remains dry and at the same time protected against corrosion. For most final surface treatments, it is no longer necessary to remove the inhibitor from the product. It has excellent compatibility with paint systems and other methods of corrosion protection. When using some preservative oils, it is only necessary to perform a preliminary test to see if the additives contained in them somehow react with the inhibitor. But since this liquid replaces the use of preservative oils, their use is only relevant in tropical climate conditions or sea transport, assuming long-term contact of the product with water or salt water. When using the product, make sure the workplace is well ventilated both for work safety and fire protection reasons, as this liquid is class 3 flammable.

Peter Stuchlik

MSc, PhD., CTex ATI