Rust stains

 

on calcareous materials

 

Rust evidently spoils stone materials by forming concentrated red-brown stains or large yellow or yellow-ochre halos having an all the more marked effect when the base material has lighter colouring; the result on white materials is often devastating. Rust stains are formed by various mechanisms and contact with metallic materials is one of the most frequent cases: the metal (usually iron or iron alloys) oxidises in the presence of humidity and oxygen (air) and thereby transfers its ions to the stone, forming rust that is absorbed and retained in the porosity of the material itself. In particular, the oxidation process initially forms ferrous and ferric ions that migrate into the film of humidity and from there to the stone itself, where they are rapidly converted into ferric hydroxide through the action of oxygen and humidity subsequently to become rust as such. Another equally important factor is the oxidising deterioration of certain parts of the pigmentation of the material (for example, the pyritic components); they oxidise in the presence of air and humidity and cause extensive deterioration and stains. Rising damp from the base of the stone may also cause serious damage: they carry salts and iron compounds from the anchorage mortar, e.g. pyrite ash in Portland cement, sand and gravel components rich in iron oxides, or pieces of metallic iron from waste, etc. In this latter case, they form huge yellow halos and areas which are often very difficult to eliminate since they tend to re-form and regenerate because they are continually supplied from below (from the bonding agent), especially if the base is moist.

 

Then there is the example of the so-called “false iron stains” - stains and halos again having a yellow colour that are visually similar to those described above but formed by the action of tannins, tar, polyphenols, rotting products, etc. following accidental contact with leaves, sawdust, chips, cardboard, cigarette ends and organic material in general. Since they are completely different in nature, eventual elimination of this type of stain involves a different method which is not reviewed in this article.

 

Types of rust stain

 

When the rust stain formation process begins, iron hydroxide - Fe(OH)3 - is almost always formed, having a colloidal nature and a bright red colour, that in general is easily removed by delicate abrasion, better if a suitable surfactant is also used. Subsequently, a series of dehydration reactions take place involving the ferric hydroxide thus formed, with the following typical outline: Fe(OH)3 = FeO(OH)+H2O with formation of hydrated ferric oxide, Goethite with a darker colour and more compactness than the initial hydroxide that also forms crusts and deposits that are more difficult to remove. If the iron, on the other hand, is present in widespread and diluted form in the stone, it creates large areas with yellowish halos of variable intensity. In particular, more porous materials tend to promote formation of stains that penetrate more deeply into the stone by capillarity and make subsequent removal operations all the more complex.

 

Reactivity of calcareous material

 

compared to siliceous material

 

As is well known, while materials having a silica matrix (granites, for example) are very resistant to the action of acids and, in practice, can only be attacked by hydrofluoric acid HF with the formation of volatile SiF4, calcareous material is substantially made up of carbonates (CaCO3 and MgCO3) and by nature incompatible with acids. They react with acids to form calcium salts and release carbon dioxide, with consequent attack on the calcareous matrix, in accordance with the following typical outline: CaCO3+2H+ Ca+++CO2+H2O. It is also common experience that even weak acids such as citric acid (contained in citrus fruit), tartaric acid (contained in wine), acetic acid (contained in cooking vinegar), etc. visibly attack marble tables, kitchen tops, trusses, shelves, etc. Acid attack on limestone is all the more evident and visible the more highly polished the surface: for example, the dissolution of part of the extremely thin “polished crust” of a white marble forms distinct zones of contrast when reflecting the light that make such attack all the more evident. Eve a significantly alkaline environment may also damage limestone, especially over long reaction times, with the formation of calcium hydroxide. Inasmuch, prevention of such attack on calcareous stone by treatments using water-based solutions must be performed in a neutral or weakly alkaline environment, normally having a pH value between about 7 and 9.

 

Rust stain

 

elimination techniques

 

• Use of specific acids

 

Even if acids attack limestone, as previously described, their effect may be limited when specific acid solutions are used that form calcium salts that resist dissolution and thereby inhibit the entity of attack on the calcareous matrix itself. An example is HF, H2C2O4, H3PO4 whose calcium salts ensure rather low solubility and, consequently, their attack on a calcareous material is not deep - albeit, still clearly visible when the surface of the marble is polished. Solutions of these acids are therefore used to eliminate traces of rust from limestones with rough as-cut finishing, honed or in any case subject to re-polishing. Naturally, in order to minimise the effect of this type of acid, certain expedients are necessary which help improve the protection of the limestone, such as preliminary imbibition of the material with water, subsequent rinsing and the use of suitable concentrations of these acids. More information about this topic will be provided later.

 

• Use of complexing agents

 

This method exploits the capacity of some compounds to bond in a very stable way (depending on pH and other parameters) with certain ions: an example is complexing agents, also known as chelating or sequestering agents. They are already widely used in many sectors: agriculture, detergents, pharmacology, etc. and iron is one of the ions most studied in the field of chelating agents. In particular, a great many organic and inorganic compounds form stable complexes with iron but our focus here is on those chelating agents that form stable complexes with iron in a neutral or slightly basic environment since the nature of the calcareous matrix would not tolerate the action of an acid environment (as extensively described above). There is then the by no means negligible problem that the calcium in limestone itself may be complexed and when this takes place in a marked manner it involves damage of the calcareous matrix that, on the other hand, we only intended to clean and protect. Then there are other problems associated with the cost and toxicity of the product used (by way of example, using cyanide as a chelating agent would be strongly unadvised for evident reasons of safety…). It should also be noted that the acids indicated above (HF, H2C2O4, H3PO4) themselves also form complexes of some stability and that their action is also enhanced by this effect. However, this all takes place in an acid environment.

 

Many products of varying stability can be used to complex iron in a neutral or weakly alkaline environment; they include: bi-sodic or tri-sodic EDTA, ammonium oxalate, etc. and certain organic oxydrilate acid salts such as: sodium citrate, sodium tartrate, etc. Unfortunately, the low solubility of Goethite and the by no means high stability of the complex with iron of such chelating agents in an alkaline environment makes their exclusive use very difficult (or in any case too slow, since many successive treatments would be necessary), without considering the corrosion of the limestone itself following complexing of the calcium.

 

• Use of redox systems

 

Since the iron in rust is in a +3 oxidation state and since ferrous hydroxide (where iron has an oxidation state of +2) is much more soluble than the corresponding ferric hydroxide, we hypothesised preliminary action to reduce iron from +3 to iron +2 so that removal, in a neutral or weakly alkaline environment, would undoubtedly be easier. This hypothesis then involved various reactive reducing agents such as: sodic thyosulphate, sodic thyoglycolate, sodic ferricyanide, etc. etc. and various formulations were provided with relative operating conditions that were often far from simple. Some authors have also proposed treatments performed in a controlled environment (e.g. heated) that, however, are difficult to implement on flooring and other similar everyday structures. In other cases, the reducing agent was associated with an strong complexing agent (sodic EDTA, citrates, sulphonates, etc.) with the task of immediately sequestering the iron +2 formed, thereby preventing its subsequent re-precipitation and eventual rapid re-oxidation. In other cases, the use of absorbents was proposed (sepiolite, attapulgite, talc, waterproof cotton wool, etc.) to impede or limit resorption of iron +2 by calcareous stone itself. In still other situations, a specific surfactant was also used to improve the wettability of the stone and the penetration of the reagents themselves and, consequently, their activity. More refined and complex formulations also introduced the use of pH buffering agents to avoid changes in acidity/alkalinity and consequent loss of activity of the reducing agent and the complexing agent alike.

 

In any case, it must be borne in mind that the treatments envisaged and implemented for works of art and high prestige artefacts are often unsuitable for ordinary installations (flooring, cladding, staircases, furnishing and kitchen parts …) for reasons of cost, adaptability, time, eventual toxicity and so forth.

 

• Preliminary pre-treatments

 

In order to ensure successful outcomes for the rust stain treatment and removal process, the stained part of the stone must be easily reached by the reactive solution and subsequent re-precipitation of the dissolved iron must be prevented. In practice, the following is necessary: 1) carefully clean and de-grease the surface to be treated, 2) saturate the stone in advanced with clean water and then eliminate excess water, 3) add a specific surfactant to the treatment solution to improve wetting and penetration, 4) use a valid solid sorbent agent (sepiolite, attapulgite, talc, etc.) or thickening agent (glycerine, xantana gum, cellulose esters, etc.) to increase contact time and facilitate the extraction of the solution from the stone, 5) if evaporation of the solution is too rapid, cover with a recipient or sheet of plastic, 6) at the end of the operation, eliminate ant absorbent and rinse thoroughly with distilled water, 7) if necessary, repeat the operation several times, if possible using dilute solutions (and even lukewarm, if possible).

 

• Final remarks

 

Accurate experimental tests in the laboratories of the Department of Chemical Engineering, University of Pisa, have highlighted that the removal of rust, in the form of evident red-brown stains of Goethite, from white calcareous materials (e.g. Carrara marble) can be performed relatively quickly, without damage to the surface and its eventual shine and polishing. The elimination of widespread stains (the so-called “yellow halos”), on the other hand, is a little more complicated and slower and the treatment must be repeated several times to achieve a fully acceptable effect. The elimination of halos, however, becomes very demanding in situations where the stains themselves re-form even only a few weeks after their elimination (flooring in general); the removal of certain flooring tiles, in such cases, revealed the presence of a traditional cement-based bonding agent (Portland cement) which was viewed as the cause of re-formation of these stains and halos.

 

Studies are by now at an advanced stage to finalise a suitable protective agent that prevents, or at least evidently slows down, the formation of rust stains on marbles and calcareous materials in general.