Jos Abbing
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Experiencing harsh industry applications for many years now, I have seen several unpleasant results of corrosion. The global corrosion costs are huge, more than 2000 billion Euros according to NACE. Almost 60% occurs in industrial use, with the chemical, process, and oil & gas industry accounting for the majority share.

These types of industries are coping with demanding environmental and process conditions in production and operation. This includes associated services, such as in heat-transfer systems, transmissions, and distribution and storage of gases and liquids. Prevention or control of corrosion by the use of inhibitors often proves to be an economic solution.

Using a low flow control system can help you dose more accurate amounts of corrosion inhibitors. Accuracy is crucial here; it greatly influences the efficiency and minimizes environmental impact of an inhibitor system.

General corrosion factors

In fact, all metals have a tendency to corrode or dissolve to some degree. Corrosion is a natural process converting metals to a more chemically stable form. The main process medium and the environment have a major impact on corrosion factors such as oxygen, water content, acidity levels, temperature and other factors.

Image description Corrosion Factors

Influencing these main drivers allow corrosion to be stopped or slowed down sufficiently and it is here that inhibiting can play an important role.

Desgining in corrosion resistantance by selection of best compatible material and combinations, additional material thickness, and application of protective coatings may have an initial technical preference to inhibiting. Additionally, metal damage by erosive particles, fatigue, mechanical stress or cavitation may cause corrosion processes which cannot be controlled sufficiently with the use of inhibitors.

However, prevention or control of corrosion by inhibiting often proves to be an economic solution in lots of other situations, improving life time and operational costs with minimum environmental impact. Some relevant examples are to follow.

Examples of metal corrosion

Corrosion can have different drivers and causes:

Image description Anode-cathode principle

Galvanic corrosion requires two different metals that are in electrical contact. When exposed to an electrolyte, a migration of ions from the anode to the cathode causes a release of free electrons. The more noble metal (cathode) is protected and the more active metal (anode) tends to corrode.

Electrochemical corrosion, involving the release of electrons of anodic parts, is related or involved in a lot more corrosion processes, such as concentrated cell (crevice) or pitting corrosion.

Another example is chemical corrosion, which is often induced by strong oxidants, and may not be accompanied by the flow of electric current.

Biological corrosion is caused by the presence and growth of micro organisms. Their direct presence or their corrosion product caused by metabolic activity of the organisms damages the metal which can also lead to pitting or crevice corrosion.

Inhibitor classifications

The task of an inhibitor substance is to slow down or prevent the damage caused by corrosion to acceptable levels. Most corrosion inhibitors used are multi-component mixtures. Below some important examples for (liquid phase) inhibitors.

Image description Inhibitor classification

Environmental or scavengers inhibitors control corrosion by reducing or removing the corrosive properties in the medium, often involving oxygen reduction.

Interface inhibitors form a protective film on the metal, isolating the metal from the corrosive medium.

Anodic inhibitor will facilitate the formation of passivation layer blocking the anodic process. The critical concentration of the inhibitor is important to secure effectivity and to prevent corrosion acceleration caused by a too high concentration of inhibitor.

Cathodic inhibitor will decrease the corrosion rate by reduction of oxygen concentrations or increase in the over potential of hydrogen liberation (poison) and precipitate (deposit) on specific cathodic areas (precipitator), forming a protective film.

Mixed or organic inhibitors can moderate both anodic and cathodic principle e.g. by adsorption, chemisorption and film formation. An adsorption processes (physical) is relatively quick but are also more easily removed from a surface, requiring careful control. Chemisorption is a chemical adsorption process, caused by a reaction on an exposed surface, creating an electronic bond of a chemical on the adsorbed surface. The higher the concentration the greater the protection with a limit to a maximum. By exceeding the maximum concentration, corrosion acceleration is often observed.

Enabling smarter dosing control

A corrosion inhibitor system will add small concentrations of (bio) chemicals into the process. The effectiveness of an inhibitor system greatly depends on the correct injection amount. The correct injection amount is also influenced by the environmental and process conditions.

Image description Ex Zone 1 scale inhibitor with Coriolis mass flow controller

The required weight fraction of traditional mix of biocides, other inhibitor substances, agents, surfactants and pH regulators may vary e.g. between 0.001 and 0.1 weight %. The inhibiting system may inject in parts per million (PPM) to achieve low concentrations to be effective. Both continuous and shot dosing systems are used, based on the situation.

Traditional methods often involve manually tuned piston pumps with check valves. Verification of flow, by changing the stroke length, is often carried out empirically with stop-watch and graduated gauges. This traditional approach makes it virtually impossible to actively compensate to changing process conditions, such as temperature changes (caused by day/night). The result may be the worst-case flow setting, increasing chemical use, environmental impact and also cause over-dosing (!) of chemicals under normal operation conditions.

Accurate flow control

Accurate flow control enables cost effective applications with less environmental impact. High accuracy and high turndown ratio is achieved based on pure mass measurement with mini CORI-FLOW. This mass flow meter can also directly control valves and pumps by on-board PID control and can be further optimized with PLC and HMI control extending both performance and flexibility.

Coriolis dosing system

Our Coriolis dosing system approach, with digital communication, enables real-time monitoring, control and logging of injection rates. This allows online checking of flow rates and instantaneous re-setting of the required flow rate. Asset management and preventive maintenance is supported with several active diagnostics such as on-board status alarms enabling, steering monitoring, density alarm changes, single or multi point totalization for costs calculations, empty tank alarm, and pump protection shut down.

Bronkhorst has been supporting field applications and R&D research projects with extensive know how on low flow fluid handling. The ongoing research for even more environmental friendly solutions, such as biodegradable based inhibitors, is gladly supported by us.

Corrosion inhibitors are also used in other kind of industries such as public water systems. Check our blog about how phosphates are added to our drinking water as corrosion inhibitor to prevent the leaching of lead and copper from pipes and fixtures.

Bronkhorst Coriolis mass flow controllers can be an ideal solution for corrosion inhibitors. Have a look at our Coriolis product line.