How do I control corrosion

corrosion (corrodere> destroy, eat away, gnaw) and Corrosion protection are topics that are common in many areas in the Metal processing (Industry and craft), but also at Concrete structures and cement-lined pipelines and components (sewers) are of great importance and cause enormous damage. The Manifestations the corrosion are varied and expose an extensive Expertise ahead to corrosion to avoid or Corrosion damage to remove.

The basics of Corrosion processes are mainly based on
  • chemical processes
  • electrochemical processes
  • metal-physical processes (e.g. "tin plague"> disintegration of the tin (e.g. soft solder) at very low temperatures due to lattice transformation)
Under corrosion one understands the attack and the destruction metallic materials Reactions With Active ingredients from the Surroundings. The emerging Corrosion products (e.g. rust [FeO (OH) iron oxide hydroxide] for steel / sodium carbonate cuprate (II) trihydrate for copper / Al (OH)3 in the case of aluminum) soluble, insoluble but also firmly adhering be. The surface of a metal is damaged by the corrosion up to the point of total material degradation. In addition, z. B. the cross-sections of Pipelines due to the large volume corrosion products narrowed up to the total Sealing.

 

 

The Manifestations the corrosion to Art of Abortion and mechanical stress are:
  • A Contact corrosion is formed by a galvanic element at the contact point of different metals.
  • A Erosion corrosion (Cavitation corrosion [punctual or small-area erosions]) arises from Cavitation and Implosions at narrow points (e.g. pipe bends, unburred pipes, indented constricted pipe cross-sections) or in fittings and pumps in connection with high flow velocities of liquids. A erosion can also through solid substances (e.g. sand) as a result of a Abrasion be evoked.
  • A selective corrosion (intergranular / transcrystalline corrosion) occurs at the grain boundaries in ferritic Cr and austenitic CrNi steels (stainless steel). The structure can be weakened so much that a Grain disintegration entry. When the temperature rises to 400 to 800 ° C, chromium carbides precipitate at the grain boundaries of the metal structure. Leaching in acidic liquids then reduces the amount of chromium in the alloy required for passivation.
  • A erosive corrosion (Roast - Even surface removal, even corrosion) is caused by water and / or contaminated air, with metal being removed parallel to the surface.
  • A Pitting corrosion (Pitting) is caused by punctual damage to a protective layer on the material wall (pores in coatings or by chloride attack on passive layers). Punctual, crater-shaped or pin-prick-like depressions arise.
  • A Crevice corrosion forms between structural particles or in gaps in the metal, as an electrolyte is held in place by capillary action. Large potential differences between the different metals create an electrochemical series of voltages.
  • A Stress corrosion cracking (SPRK) only occurs when a workpiece or component Tensile stresses contains, the material used has a SPRK sensitivity and a special medium acts. Cracks develop under mechanical stress in the presence of corrosive liquids (e.g. containing chloride and strongly alkaline) in austenitic CrNi steels (stainless steel).
  • AVibration corrosion cracking(SwRK) can do with everyonemetallic material in each Electrolytes occurrence. The Fractions arise exclusively through transcrystalline cracksthat arise perpendicular to the acting principal normal stresses.
  • A Filiform corrosion (thread-like corrosion) occurs in coated metals and alloys (steel and aluminum alloys). Corrosion is caused by a high relative humidity (60 to 95%), contamination of the surface, sufficient permeability of the coating for water and oxygen, starter salts (e.g. NaCl, chlorides [de-icing salts], salty sea air) and mechanical damage to the coating. In the coastal areas in western Europe there are problems in particular with aluminum alloys in construction (facade cladding).
  • A Condensation corrosion caused by dust or extraneous rust on low-alloy and unalloyed steel, as well as on stainless CrNi steel (stainless steel). The surface is severely attacked.
  • A Dew point corrosion (Acid condensate corrosion) occurs when the dew point is not reached, whereby water or acid (e.g. from combustion gases in the boiler or exhaust pipe) condense on metal surfaces and lead to local or extensive damage (uniform surface corrosion and well corrosion).
  • A Standstill corrosion(Oxygen corrosion) occurs on unalloyed steels in systems or system parts that are exposed to static liquids for long periods of time. Here, non-protective, voluminous corrosion products form in oxygen-rich aqueous media. An increasing concentration can cause the rate of corrosion to increase.
  • A microbiologically induced corrosion(MIC) is created in Investmentsthat often stagnant or slowly flowing water to have. The water can do this too biocide free be because yourself Microorganismsthat adhere to the pipe walls in contact with water, grow and multiply at low temperatures. Individual settlements form a primary film up to macroscopic growth. These processes are also called "Biofouling"or also under the term"Biocorrosion".known.
 
A Contact corrosion is formed by a galvanic element at the contact point of different metals and is dependent on
  • Electrolyte composition
  • Electrolyte resistance
  • Potential difference
  • Area ratio of the more noble to the less noble metal

Contact corrosion is also called one accelerated corrosion one metallic area, the corrosion element consists of a pair Metal / metal or Metal / electron-conducting solid. Here, the accelerated corrosive area is the anode of Corrosion element. This type of corrosion occurs frequently in Mixed installations (e.g. copper / steel) or at Mixed components, (e.g. light metal with steel elements or surface-coated components).
The Ion current can of all Electrolytes (conductive liquids) (e.g. drinking water or condensation water). The Electron conduction takes place on Contact points the solid body (e.g. screw connection, screw head and clamped parts). In many cases, the moisture in the air is enough to cause contact corrosion.
The Strength the corrosion always depends on the Potential difference, the Area ratio of each other paired metals or Components and the conductivity of those covering them Electrolytes from.

A different way the Contact corrosion is a Local element formation

- between various alloy components on the metal surface
- between Foreign metal particlesthat have been washed into the pipeline or pressed into surfaces
- at a Emergence of anodic and cathodic areascaused by opposites in the metallic material (e.g. at cold-formed points with different degrees of deformation [e.g. screws with half threads]).

A Contact corrosion can especially with brazed plate heat exchangers (Drinking water heater - fresh water station) stainless steel occur. Contact with the more noble stainless steel can result in a resolution of Copper solder come. It can leak to the outside or to the side of the primary heating medium. Also in Hot water pipes out hot-dip galvanized steel are serious damage from Pitting corrosion possible caused by the corrosion of the Copper solder copper ions going into solution.

Crevice corrosion in a press connection on stainless steel pipes
A Crevice corrosion forms between structural particles or in gaps or cracks in the metal, in which an electrolyte is held by a capillary effect. Large potential differences between the different metals create an electrochemical series of voltages.
In columns (Damage to surfaces) and Dead dreaming (e.g. in press connections) metallic materials (Pipelines, connections) that form protective oxide layers, the liquid reacts with the surface of the material and reduces the oxygen content. Here hydrolyze the emerging Corrosion products, the pH drops and Anions how chloride ions can immigrate. As a result, the oxidic protective layer can no longer be maintained and severe corrosion occurs in the gap. In this gap arises because of the very low volumes quickly a aggressive liquid.

Further positionsCrevice corrosion forms on the threads of screw connections, spaces in seals and between shafts and protective sleeves.

But even with materials that are extremely sensitive to hydrofluoric acid and fluoride, slight roughening can occur under PTFE-containing flat seals after a long period of operation.
A Pitting corrosion (Pitting) is created by a punctual injury one Protective layer on the material wall (pores in coatings or due to chloride attack on passive layers). It arise punctiform, crater-shaped or pinprick-like indentations.
Pitting and pitting corrosion
In the case of pitting corrosion, the electrolytic metal removal only at small areas (locally limited) on the pipe wall. The emerging Holes are commonly sharply delimited and the environment is not or only weakly attacked. Since the full wall thickness is still present in addition to the holes, it looks like a hole made with a drill. The Material removal can very fast occur. In addition to pitting corrosion, in most cases there is also a slowly progressing one Well corrosion instead of.

The emergence of Ventilation elements (Evans element) is the basis for pitting corrosion unalloyed and low alloy steel.

When a Concentration gradient of oxygen is available, then the iron goes to poorly or not ventilated places (Local anode) in solution. The electrolyte solution is at the local anode angry, as hydrolysis of the iron's corrosion products takes place. In well-ventilated areas (Local cathode) oxygen is reduced to hydroxyl ions and the electrolyte solution alkaline. Through the Alkalization can become a spontaneous Passivation of steel come and through that excretion of Corrosion products (Iron (III) hydroxide), which also forms cover layers, prevents corrosion on the local cathode. In addition, locally different ones act pH values with the different oxygen content even more intensely on the progress of corrosion.
A selective corrosion (intergranular / transcrystalline corrosion) occurs at the grain boundaries in ferritic Cr and austenitic CrNi steels (stainless steel). The structure can be weakened so much that a Grain disintegration entry. When the temperature rises to 400 to 800 ° C, chromium carbides precipitate at the grain boundaries of the metal structure.Leaching in acidic liquids then reduces the amount of chromium in the alloy required for passivation.
Crystallites in one Alloy structure (e.g. copper and zinc crystallites in brass) react with one another on the surface via a film of water because they consist of compounds with different electrochemical potential. The Corrosion attack preferably arises along certain structural areas of the material.

After this Area of destroyed structure a distinction is made:
- Intergranular corrosion The destruction runs along the grain boundaries (stainless chromium and chromium-nickel steel, nickel, copper, zinc and tin alloys). Alloy components are separated out.
- Transcrystalline Corrosion The destruction (electrical voltage differences and strong tensile stress) runs through the grains of the structure. This type of corrosion is also under "Stress corrosion cracking"classified.

The progress this corrosion can only occur under the Magnifying glass recognized because theselective corrosion in Grain size range occurs. Paragraphs can then be seen on the corrosion layer.

The Knife line corrosion is a Special shape the intergranular corrosionthat after the welding of stainless steel (stabilized carbon steel - 12% chromium) can occur on both sides of the weld. The reasons lie in the resolution the Niobium carbides at the high temperatures and the excretion of Chromium carbides after a subsequent heat treatment (temperature range of approx. 600 ° C). At the Melting line a narrow, knife line fine, selectively attacked Zonewhat a enrichment of Carbides is due. As a result, the Chromium content from 12% fallen below and it forms no protective layer made of chromium oxide, which is supposed to protect the material from further corrosion.

In the Knife cut corrosion is it a Interfacial corrosion. This occurs with brazed ones Stainless steel components (e.g. heat exchanger), which e.g. with zinc-containing solders getting produced. The decisive factor is that Contact With chloride-containing, aqueous media.
Here is the Steel surface at the edge and under the Solder joint attacked. The Risk of corrosion is reduced by soldering with Zn-free solders and fluxes. With furnace soldering (without flux), e.g. with copper solder, there are no cases of knife cut corrosion. Soldered connections with Ni-base solder are safest.

Corrosion damage to soldered plate heat exchangers - G. Pajonk / Material Testing Office NRW



External and internal corrosion with rust and magnetite sludge
A erosive corrosion (Roast - Even surface removal, even corrosion) is caused by water and / or contaminated air, with metal being removed parallel to the surface.
The even surface removal the material surface is mainly due to chloride- or Liquids containing sulfur dioxide evoked. This type of corrosion is caused by U.N- and low alloy steels also as Roast designated. If the Erosion rate under 0.1 mm / a is then the material as sufficiently resistant viewed against corrosion.
Also stainless steels can rust. The reasons here are that there are no Passive layer forms or the passive layer has been destroyed. Therefore, a high level of cleanliness must be ensured during processing and the processed surfaces must be cleaned of all residues.
rust
(x FeIIO * y Fe2IIIO3* z H2O [x, y, z positive ratios])

The surface of Ferrous metals (Iron, steel) goes with oxygen and water or. humidity a chemical compound (oxidation) a. The product is rust (Iron oxide [hydrous iron oxide]). Some Fabrics (e.g. salt [salt water, salty air]) accelerate the Rust formation.
Rust causes the material porous and if oxygen and water continue to be present that decomposition advances. With increasing decomposition, the material becomes more porous, brittle, and may burst due to the larger volume, up to total destruction.

Blocked distributor connections

rustoxidationcorrosionis not the same.
The oxidation is a Electron donation of a substance. These electrons have to be taken up by another substance (reduction). rust is a special shape of oxidation. Must be an oxidation Not lead to rust. So z. B. in zinc or aluminum by reacting with oxygen wafer-thin oxide layer. This thin oxide layer protects the metals before another chemical reaction. That's why a distinction is made between Ferrous metals (which can rust) and Non-ferrous metals (which do not rust). Of corrosion (to eat) one speaks generally of a chemical reaction of metals with their surroundings, leading to a impairment the function or destruction leads.

Rust film (Iron dust) is one incipient corrosion of ferrous metals. What is meant is one thin layer of rust on a surface that is easy to clean. Flash rust can lead to further rusting if it is not removed or the surface is not treated.
stainless steel available in different ways and is not to be equated with stainless or stainless steel, but has under normal environmental conditions a sufficient one Corrosion protection, but can under certain conditions roast. The most common problem is that collection of Rust film or contact with rusty objects on a Stainless steel surface.

White rust can e.g. when there is a lot of moisture Zinc surfaces form. If the moisture is no longer available, the white rust does not spread any further. So should z. B. galvanized sheets weatherproof (dry) stored because of the Corrosion protection reduced if it works longer.

Corrosion on stainless steel pipelines

A Erosion corrosion (Cavitation corrosion [punctual or small-area erosions]) arises from Cavitation and Implosions at narrow points (e.g. pipe bends, unburred pipes, indented constricted pipe cross-sections) or in fittings and pumps in connection with high flow velocities of liquids. A erosion can also through solid substances (e.g. sand) as a result of a Abrasion be evoked.
Erosion corrosion or Cavitation corrosion is a selective or. small areaRemoval one Metalby a relative high flow rate occurs on a metal surface. It often occurs in Elbows and Pipe bottlenecks (Dents, screw connections, non-deburred interfaces) that influence or increase the direction or speed of flow. The basic mechanism of this type of corrosion is that one continuous flow of liquid the protective film or Oxide layer from a metal surface away. She can get through Suspended matter (Rust, sand) can still be enlarged. This effect is also used in the Sandblasting used, with even stubborn deposits at relatively low flow velocities can be removed. Due to the lack of a protective layer, the Metal surface unprotected and is from corrosive medium (Liquids with an unfavorable pH value, oxygen and electrolyte content) attacked and corroded by the friction the liquid and the emerging Microbubbles (Cavitation - implosions), which is especially important in Fittings and pump can occur in heating, solar and cooling systems.
Soft metal alloys (e.g. copper or aluminum) particularly vulnerable. With an increase in the flow rate, the thermal power, e.g. B. in heat exchangers, but also increases the risk of erosion corrosion. The following Flow velocities should be at cold water not be exceeded:
Cast iron - 3.0 m / s; Stainless steel - 4.6 m / s; Copper - 2.4 m / s; Aluminum; 1.8 m / s
With increasing temperature that increases risk the Erosion corrosion. So z. B. at copper a flow velocity of 2.4 m / s for cold water and can be used at 60 ° C warm water 1.5 m / s do not exceed. In water systems that are operated continuously above 60 ° C, the flow rate should not be higher than 0.91 m / s be. This is also the reason why Circulation lines balanced should be.
A Stress corrosion cracking (SPRK) is caused by the formation of cracks under internal stress or under mechanical stress in the presence of corrosive liquids (e.g. containing chloride and strongly alkaline) e.g. B. with austenitic CrNi steels (stainless steel).
Cracks in fittings eccentrics; Inner surface partly forged, partly turned
Source: Dipl.-Ing. Wolfgang Allertshammer

A special type of contact corrosion is Stress corrosion cracking (SPRK), which only occurs when a workpiece or component Tensile stresses contains, the material used has a SPRK sensitivity and a special medium acts. After a Exposure time form Cracksthat are only visible under with a microscope from which visible Cracks appearthat develop perpendicular to the direction of the tensile stresses and whose course is both transcrystalline and intercrystalline. This damage is only noticed after a wall breakthrough. Austenitic chrome-nickel steels and Copper-zinc alloys (Brass) are particularly vulnerable. Cast materials of the same kind are more resistant than the corresponding ones Rolling and Forging materials.

AVibration corrosion cracking (SwRK) can do with everyone metallic material in each Electrolytes occurrence. The Fractions arise exclusively through transcrystalline cracksthat arise perpendicular to the acting principal normal stresses. A distinction is made between one SwRK
  • in the activeStatusthe cracks are mainly caused by corrosion troughs on the surface. The cracks appear next to each other and show a fissured fracture pattern with corroded crack flanks.
  • in the passive state the cracks arise from the damage to the passive layer from inside the material. This results in smooth, slightly branched cracks that do not form any corrosion products.
The lifespan one stressed component (Tension and pressure, rotating and alternating bending) depends on the corrosion resistance of the material to the surrounding medium, the degree of its activation through plasticization, the level of mechanical vibration stress and its frequency.
A Filiform corrosion (thread-like corrosion) occurs coated metals and Alloys (Steel and aluminum alloys). The corrosion is caused by a high relative humidity (60 to 95%), Impurities the surface, sufficient Permeability the Coating for water and oxygen, Starter salts (e.g. NaCl, chlorides [de-icing salts], salty sea air) and mechanical damage the coating. In the coastal areas in western Europe there are problems in particular with aluminum alloys in construction (facade cladding).

Under the Coatings of a metal are created by one electrochemical process thread-like traces on the metal. Here the Thread head the role of active anode, there the corrosion of the metal takes place with simultaneous concentration of the surface electrolyte (moisture). With the involvement of oxygen run in the Thread trunk the cathodic reactions. The corrosion is stimulated by the Potential difference between the thread head and stem.
This corrosion is the strength of the material not endangered, since the penetration depth of the damage is not great (<40 µm). So it's more like a Beauty problem the coated surface, which can result in significant repair costs. Efficient methods of protection are not yet known.

Filiform corrosion

A Condensation corrosion (Condensation corrosion) is caused by dust or extraneous rust on low-alloy and unalloyed steel, as well as on stainless CrNi steel (stainless steel). The surface is severely attacked.

The Condensation corrosionthat are due to a high humidity (> 60% relative humidity) with Impurities from the air enriches and enriches rough spots of unprotected Pipe walls forms is the most common atmospheric corrosion. Here have an effect strong temperature and Fluctuations in humiditywhich lead to the constant formation and drying of condensation water droplets, have a particularly strong effect on the progress of corrosion on the surface.

The different Impurities in the Ambient air and at the Pipe surface have different effects on corrosion.

  • dust and soot Forms condensation nuclei on the surface and act like a sponge that binds the humidity. Aggressive salts can then be removed from the dust here. In addition, soot particles have the property of acting like a cathode on iron, which leads to the formation of local elements.
  • Sulfur dioxide forms iron (II) sulfate by hydrolyzing sulfuric acid and iron oxide as it acts as a catalyst. The sulfuric acid that forms destroys the protective film on the surface of iron.
  • Chlorides act like sulphates, but are only important in the immediate vicinity of the sea and in industrial areas with acid or chlorine processing plants.
  • Nitrogen oxides act like sulfur dioxide and can intensify the effect of sulfur dioxide due to the oxidizing properties.
  • ammonia and Amines only have a corrosive effect in the vicinity of farms because ammonium salts are formed here. In the case of copper and aluminum parts, these are strong corrosive poisons that can lead to the rapid destruction of the component.

 

Acid condensate corrosion in the boiler

 

Acid condensate corrosion in the reheating box of a warm air tiled stove
A Dew point corrosion (Acid condensate corrosion) occurs when the dew point is not reached, whereby water or acid (e.g. from combustion gases in the boiler or exhaust pipe) condense on metal surfaces and lead to local or extensive damage (uniform surface corrosion and well corrosion).
If with a condensation by a Falling below the dew point in one boiler Components from the fuel (e.g. heating oil> sulfur) go into solution, then one speaks of Acid condensate corrosion. The aggressiveness of Condensate on the dilution from the sulfur-containing combustion gases (concentrated sulfuric acid). Have a strong corrosive effect. But also condensates out chlorine-containing gases (e.g. burning plastics containing chlorine) very corrosive.

The Corrosion damage through the Acid condensate and Dew point corrosion usually occur Put where the Below the dew point becomes. In Heating boilers these areas are mainly in the area of ​​the heating return and in the lower area of ​​the combustion chamber or flue. But also in Smoke- or. Exhaust pipe or in Metal chimney condensation can form.

Also in the Flue gas system one Tile- or Fireplace, with sulphurous fuels (Coal, lignite briquettes) are operated, it can rust through due to years of non-use when using materials without rust protection Corrosion damage come.
Particle formation in copper gas pipes
Homogeneous, black, matt glossy layer 
Source: DBI Gas- und Umwelttechnik GmbH
Particles in the copper pipe of a solar system

A Particle formation at the Inner surface of Copper wires in Gas house installation (Customer systems) is found again and again. If the particles are in the filter at the Gas valve collect, it comes to Fault report, so to the failure of the gas appliance. The reason