The "flash rust" is a very rapid corrosive phenomenon that can occur after cleaning, sandblasting or washing a ferrous product.
This occurs when the treated surface is exposed to the air, coming into contact with humidity and oxygen, which can give rise to yellow-orange oxidation creations, mainly composed of iron hydroxide. These oxidations are not difficult to remove posteriorly but if left on the surface they will evolve into persistent ferrous oxidations. We can easily find the flash rust even just by spraying the tap water on an iron surface, after a few minutes, we will see the so-called pitting corrosion phenomenon, or spots. While in other circumstances the flash rust, for example after a washing cycle, can have the appearance of a uniform yellowish layer composed of a powder easily removable to the touch. If this corrosion remained on the piece, we would most likely have paint defects, which is why flash rust must be absolutely avoided.
How to avoid it
The attention must be paid above all to those manufactured items of iron and cast iron, or non-stainless steel, the more sensitive the metal is, the more oxidative phenomena we will have. In the second instance, we must try to limit acid washes on these surfaces because this corrosive process would be accelerated exponentially, however, since it is often essential to use acid pickling to remove strong oxidations, it is necessary to adapt important precautions, or add corrosion inhibitors in the pickling and rinse with neutralizing and demineralized water. The use of tap water is in fact harmful for iron as it provides ions such as chlorine which are strongly oxidizing.
Using pre-treatments or alkaline rinses, flash rust or other oxidative phenomena will not occur.
To limit the flash rust, it is also important to act on the acidity of the bath, on the temperature and on the dripping time of the pieces, before they are dried in the ovens. By intervening on these parameters it is usually possible to limit this type of reoxidation. The last trick is obviously to paint or protect the pieces as soon as possible in order to block oxidative processes.
Important in formulations, but not very useful if not balanced with the acidity and constitution of the product used. A too acid and aggressive product can hardly be managed by a corrosion inhibitor and the reoxidation will take place after the extraction of the products from the bathroom. It is therefore necessary to use buffered acids or mixed with organic acids that help mitigate corrosion. Given the usually high cost of specific inhibitors, the percentage in formula should never be too high. If too much inhibitor is needed to handle the flash rust, it means that the formula is unbalanced and too aggressive. In addition to the specific inhibitors on the market, suitable for various applications and metals, we can also consider using traditional always functional raw materials, such as some glycols, benzalkonium chloride, benzotriazole, some organic acids such as citric, amines etc.