assignement corrosion

The most suitable material for our application would be AISI 316, but in some parts of the plant because of

economic reasons we can also use AISI 304.

Given the previous consideration, the materials we will use in every part of the plant in contact with the

product is SS, whose main corrosion concern is localized corrosion phenomena, in particular pitting caused

by a critical threshold of chlorides. These species can be introduced during cleaning and they are also present

in the milk.

1. Cleaning

Considering the corrosion risk, the concerns during cleaning arise from:

• Detergent use, since the species are alkaline or acidic (the pH range is variable according to the

specific detergent) and the suggested temperatures of the process is at about 70 °C or above.

Stainless steels are safe in these conditions as long as chlorides are low enough

• Disinfection with Cl , which can form Cl- and so very severe localized corrosion. If the dosage provided

2

by the producer is respected stainless steels are safe, otherwise risk comes from:

Mistake in the concentration added in the cleaning solution

- Presence of dairy residues deposited onto the metal, and this

- can be prevented, or at least reduced, by providing a

periodical maintenance (especially with mechanical cleaning)

and also designing properly the pipes and the different

components of the plant in order to minimize the positions in

which deposits can be stuck or easily retained

The chloride risk is increased in case of residual acidity from the acidic detergent previously used. For this

reason, a good solution is to wash the plant, after the deterging step and before the disinfection, with a weak

alkaline solution, to neutralize the environment before the chlorination process occurs. Notice, that these

concerns are mainly related to the initiation step of the localized attack, since the cleaning occurs seldomly.

2. Tanks

Indoor tanks are made in stainless steel AISI 304. They are often placed outdoors to save on building costs.

In this situation they have a double shell with a minimum of 70 mm mineral-wool insulation in between. The

outer shell for economic reasons is usually made of mild steel and coated with anti-corrosion paint: this is a

very important request because carbon steel is at a higher risk of damage from atmospheric pollution.

Carbon steel has a corrosion rates range from 20 µm/y in rural environments up to 200 µm/y in industrial

environments. In the worst conditions, that is a high corrosivity category (C4), so a polluted urban area, it is

possible to approximate the corrosion depth after 20 years of use of the tanks. We use the following relation: