Maritime and offshore conditions as a challenge

Offshore platforms, port installations and ships operate in one of the harshest corrosion environments in the world. The combination of salt water, high humidity and mechanical loading from waves permanently attacks metals. Engineers in shipbuilding and in the offshore industry must take these factors into account in every design.

Sea water accelerates corrosion processes many times over compared with fresh water or dry air. The high salt content makes it a perfect electrical conductor. This property enables electrochemical reactions that continuously attack metals.

Salt water and atmospheric influences on metals

Sea water acts as a highly conductive electrolyte that considerably accelerates corrosive processes. The dissolved salts, in particular sodium chloride, increase the electrical conductivity 100-fold compared with distilled water. At the same time, the oxygen dissolved in the water supplies the necessary reaction partner for metal oxidation.

Galvanic corrosion occurs particularly frequently in shipbuilding. When steel hulls come into contact with propellers made of copper alloys, a galvanic cell is created. The less noble metal – in this case the steel – corrodes at an accelerated rate, while the more noble copper alloy remains protected.

Chloride ions from sea salt break through passive protective layers on stainless steels. They penetrate microscopically small irregularities in the metal surface and initiate pitting corrosion. This form of corrosion is particularly insidious because it often remains barely visible from the outside while the material is weakened from within.

Salt-laden spray is carried several kilometres inland by the wind. Even structures far from the coast suffer from this salt load. The salt crystals settle on metal surfaces, draw moisture from the air and create ideal conditions for electrochemical reactions.

Corrosion protection for maritime installations therefore requires multi-stage protective concepts. Coating systems form the first line of defence against salt water and spray. In addition, sacrificial anodes are used, which corrode in place of the structure to be protected.

Corrosion of offshore structures and underwater installations

Corrosion of offshore structures appears in different zones with different mechanisms. The underwater area is in constant contact with sea water, but the corrosion rate there often remains moderate. The limited access to oxygen at greater depths slows down the oxidation processes.

The splash zone is regarded as the most aggressive area on offshore structures. Here, wetting and drying alternate constantly. Salt water penetrates cracks and crevices, evaporates and leaves behind concentrated salt deposits. At the next wetting, highly concentrated salt solutions form that attack metals extremely quickly.

Offshore wind turbines are a prime example of the challenges in maritime environments. Their foundations are immersed several metres in the sea water, while the tower is exposed to the salt-laden atmosphere. The transition areas between water and air require particularly intensive protective measures.

Oil platforms and their pipeline systems contend with additional corrosion factors. Inside, the pipes often transport corrosive media such as sour natural gases. Outside, salt water and marine organisms attack the material. This double loading requires careful material selection and regular inspections.

Cathodic protection has become established as the standard method for the corrosion protection of maritime installations. In this method, sacrificial anodes made of zinc, aluminium or magnesium are attached to the steel structure. These less noble metals corrode preferentially.

Impressed current protection is used for large offshore structures. Anodes made of titanium with a mixed-oxide coating are supplied with direct current. They generate an electric field that makes the steel structure the cathode and protects it from corrosion. Monitoring and control are computer-aided.

Special features near the coast and on the open sea

Coastal areas experience intense alternating loading from tides. Port installations, quay walls and lock gates undergo several wet-dry cycles every day. Each cycle accelerates corrosion through salt enrichment and renewed wetting.

Structures on the open sea are exposed to extreme wave loading and storm salt loading. Spray reaches even areas 20 metres above sea level. The mechanical loading from waves can damage protective coatings and expose bare metal surfaces.

Water temperature considerably influences the corrosion rate. Tropical waters at 25-30°C show twice as high corrosion rates as cold sea areas at 5-10°C. The elevated temperature accelerates all electrochemical reactions.

Flow velocity has a two-sided effect on corrosion processes. Strong currents continuously transport fresh oxygen to the metal surface and accelerate oxidation. At the same time, they can erode protective surface layers and cause mechanical wear.

Biological fouling by mussels, algae and bacteria creates micro-environments with different oxygen concentrations. Beneath the organisms, differential aeration corrosion occurs. Marine bacteria can also produce aggressive metabolic products such as hydrogen sulphide.

The selection of materials for maritime applications ranges from high-alloy stainless steels to modern composite materials. Duplex steels combine high strength with good corrosion resistance. Copper-nickel alloys show excellent resistance to sea water and prevent biological fouling. Titanium alloys offer the highest corrosion resistance, but are cost-intensive and are used above all in critical areas.

High-temperature corrosion in industrial processes

When temperatures rise, the corrosion mechanisms can change fundamentally. Industrial installations such as gas turbines, blast furnaces and power plants have to work with thermal loads that would quickly destroy normal materials. The combination of extreme heat and corrosive gases creates conditions under which even high-quality alloys can fail.

High-temperature corrosion typically occurs in environments from 400°C upwards. In this temperature range, chemical reactions accelerate exponentially. Protective layers that work at room temperature lose their effectiveness or evaporate completely.

Material behaviour at extreme temperatures up to 650°C

Metals show a completely different corrosion behaviour at rising temperatures. At room temperature, corrosion processes proceed relatively slowly. From about 400°C, however, new forms of attack occur that weaken materials quickly.

Scaling arises from direct oxidation of the metal surface. Oxygen from the ambient air reacts with the metal and forms thick oxide layers. These layers can break open due to thermal stresses and expose the underlying material.

Sulphidation attacks materials when sulphur-containing gases are present. Sulphur compounds penetrate the metal structure and destroy its mechanical properties. This process frequently occurs in refineries and petrochemical plants.

Carburisation damages materials in carbon-rich atmospheres. Carbon diffuses into the metal and changes its microstructure. The result is brittle areas that can crack under load.

Turbine blades in gas turbines demonstrate the complexity of these loads. They rotate at high speeds while combustion gases at temperatures of up to 650°C flow over their surfaces. At the same time, sulphur and vanadium compounds from the fuel attack the material.

Special alloying elements considerably improve high-temperature resistance. Chromium forms stable chromium oxide layers that protect the underlying metal. Nickel increases strength at high temperatures. Aluminium produces particularly dense oxide layers that act as a diffusion barrier.

Molten salts and their destructive effect

Certain molten salts are among the most dangerous corrosion phenomena at high temperatures. They arise when fuels contain impurities such as sodium sulphate, potassium sulphate or vanadium compounds. At operating temperatures these salts melt and form aggressive liquid films on metal surfaces.

These molten salts dissolve even the most stable protective oxide layers. The direct contact between molten salt and metal enables an accelerated attack. This process is referred to as “hot corrosion” and can penetrate materials in hours or days.

Waste incineration plants are particularly at risk. Household waste contains large quantities of chlorine compounds from plastics and salts. During combustion these form aggressive molten salts that attack boiler tubes.

Biomass power plants contend with similar problems. Wood and straw contain natural potassium and chlorine compounds. During combustion, molten salts are formed that deposit on hot surfaces and corrode them.

Coal-fired power plants have to deal with sulphur- and vanadium-containing deposits. Particularly low-grade coals contain high proportions of these elements. The resulting molten salts attack superheater surfaces and turbine blades.

Characteristic cavities indicate the molten-salt attack. The surface is removed unevenly, creating deep pits. Material removal proceeds considerably faster than with normal high-temperature corrosion.

Special coatings offer a certain degree of protection. Ceramic layers or aluminide diffusion coatings create barriers between molten salts and the base metal. Fuel cleaning reduces the amount of salt-forming impurities. Temperature management keeps critical surfaces below the melting points of dangerous salts.

High-temperature water in power plants and installations

High-temperature water up to 650°C behaves fundamentally differently from normal water. In steam generators and boilers, extreme conditions prevail with high pressures and temperatures.

The solubility of oxygen and other gases changes at high temperatures. Water can also attack normally resistant materials such as stainless steel. The corrosion rate increases exponentially with temperature.

Oxygen corrosion attacks boiler tubes from the inside. Dissolved oxygen oxidises the tube walls and forms magnetite deposits. These deposits reduce heat transfer and lead to local overheating.

Stress corrosion cracking threatens highly loaded components. The combination of mechanical stress, high temperature and corrosive medium generates cracks. These cracks grow quickly and can lead to sudden failure.

Erosion corrosion occurs in areas with high flow velocity. High-temperature water up to 650°C tears off oxide layers and accelerates material removal. Pipe bends and valves are particularly at risk.

The water chemistry requires precise control. Even the smallest deviations in pH value can cause catastrophic damage. Contaminants such as chlorides or sulphates considerably accelerate corrosion.

Water treatment removes dissolved salts and gases. Ion exchangers produce high-purity feed water. Oxygen scavenging through hydrazine or organic chemicals prevents oxidation. pH adjustment with ammonia or phosphates protects metal surfaces.

Austenitic stainless steels prove their worth in high-temperature water systems. They form passive chromium oxide layers that slow down corrosion. Nickel-based alloys offer even higher resistance under extreme conditions. Nuclear power plants use zirconium alloys for fuel rod cladding, as these are particularly corrosion-resistant.

Complex monitoring systems continuously check the water chemistry. Sensors measure pH value, conductivity and oxygen content in real time. Automatic dosing systems correct deviations immediately. These measures ensure plant availability and prevent costly outages caused by high-temperature corrosion.