Above Ground Storage Tanks (AST’s) are tanks or other container used for storing large quantities of water, crude oil, fuels, solvents and lubricants and various other chemicals.

The main risk connected with AST management and operation is the spilling of contained products, which may cause huge environmental damages and economical losses. For these causes, ASTs are always subject to controls and strict state/local regulations.

The main cause of tank’s failure is the corrosion of the principal tank’s components, like bottom plate, shell and inlet/outlet nozzles.

tecnocorr proposes a wide range of proven and effective solutions to manage all of these corrosion problems allowing the longest lifetime and minimizing operating costs.

Tank Bottom Plates

AST are usually laid on a basement of compacted soil or sand, occasionally reinforced or insulated using a concrete reinforced slab or an asphalt cushion.

The design of tank’s basement is crucial for a long operating life and must be performed in consideration of many different aspects.

Soil’s corrosivity estimation is generally based on electrical resistivity and other chemical properties like pH and ionic concentration.

Soil corrosivity classification

Soil data analysis Review

If high or moderate corrosion is expected for a tank laid directly on soil cushion, concrete or asphalt/bitumen sand pads are sometimes considered, but problems arising from concrete cracking, water accumulation and moisture condensation are known to lead significant generalized and localized corrosion.

ICCP for tank bottom

Impressed Current Cathodic Protection (ICCP) offers a proven and cost-effective solution for the corrosion protection of AST’s bottom plates. This electrochemical technique uses an electrical current from a transformer/rectifier unit to inhibit the chemical reactions that lead to galvanic corrosion of steel.

ICCP systems are mainly composed by a transformer rectifier powered by the main AC power distribution, an anode system to deliver and distribute the current to the tank bottom plate, junction boxes for electrical wiring and permanent reference electrodes to monitor the electrical potential of the tank bottom in respect to the soil.

Our electrical equipment is in compliance with IEC and UNI EN standards and can be supplied in ATEX certified execution for potentially explosive atmosphere areas.

Reference electrodes are the components used to monitor the electrical potential of the tank bottom in respect to the soil, a parameter that allows our technicians to determine the corrosion protection level of the structures.

Depending on basement’s soil type, resistivity and contaminant concentration, we can supply standard Cu/CuSO4 reference electrodes as well as pure zinc cell and advanced sintered electrodes with ion-trap membranes and moisture retaining systems.

For very large diameter tank, mobile reference electrodes with soil access are available.

Anodes for AST’s bottoms must deliver the appropriate amount of current even in presence of high resistivity soils, and must allow an uniform distribution over the whole bottom’s surface.

Basing on our experience, grid systems using MMO titanium ribbon and loop systems using MMO titanium wire have proven to be the most performing solutions. These anodes will ensure long lifetime, almost zero consumption rate, and an exceptional current distribution figure.

The Loop system

The loop anode system is an effective and extremely simple solution for obtaining a uniform current distribution on the tank bottom plate minimizing on-site operations. The huge advantage with loop system is that anodes are factory-assembled using our Mixed Metal Oxide (MMO) Titanium flexible anode. They must be simply placed in concentric ring configuration under the tank’s foundation and connected to DC power cables using in-line cable splicing kits.

The whole system is engineered and designed according site conditions (soil resistivity, corrosivity and temperature) and tank parameters.

No on-site welding, cutting, drilling or groundwork is required. Loop anode system is able to guarantee long-term performances even in dry or high resistivity soils.

The Grid system

Another system used to obtain a long operating life and a uniform current distribution is the grid anode.

A grid, composed using MMO-Ti ribbon or grid anodes and pure Titanium power distributor bars is laid under the tank foundation and connected using a spot welding machine for improved long-term stability.

Grid spacing is designed according soil resistivity and tank diameter.

Other anodic system

If many storage tanks are installed in the same area and close proximity (tank farms), vertical deepwell groundbed may be considered as an effective solution.

Internal Corrosion

ASTs are used to contain several kind of fluids that may have a corrosive action on the internal surfaces.

Fresh water stored for potable use, fertilization or fire prevention purposes, although being less corrosive than sea water, is well known for posing severe threats for tank’s shells and bottom plates.

Crude oil, fuels and many intermediate oil&gas processing products usually contain a certain amount of dissolved saline water (oil brine/water phase) that tends to stratify in the lower part leading to severe corrosion of the bottom and the shell.

Oil brine may also contain chemicals and contaminants such as H2S.

For these reasons, in addition to an appropriate internal coating, an internal galvanic anodes or impressed current cathodic protection system is often mandatory.

Internal corrosion represents a primary concern when handling oxidizing and corrosive fluids like caustic soda and sulfuric acid. In these cases, corrosion will lead both to the weakening of the tank’s structures and contamination of the contained fluids, and its correct management is necessary to avoid major failures.

tecnocorr proposes a wide range of solution for internal corrosion handling, including Aluminum and Magnesium sacrificial anodes, complete ICCP systems using tensioned or flanged MMO titanium anodes and anodic protection system for highly corrosive environments.