Corrosion

Reasons for the Formation of a Corrosion Element

The electrochemical processes, in which the corrosion occurs, are determined by the material, the ambient effects and the composition of the electrolytes. For the formation of a corrosions element, i.e. the generation of a potential difference, certain factors must be present:

• Material zones with electrically conductive materials at different potentials,

• a connection between these zones for exchanging charge carriers (electrons),

• completing the circuit by the electrolyte.

Corrosion elements can exist in parts, that appear to be made of “one“ material due to composition differences in the alloy or contamination.

Corrosion Types Surface Corrosion

Surface corrosion, which can be uniform or nonuniform over the entire surface, pro-duces crater type depressions. This can be countered by properly sizing the thermo-wells. Or, the material loss due to corrosion can be reduced by increasing the surface quality. Uniform corrosion is easiest to combat through use of suitable materials.

Fig. 3-28: Uniform material disintegration corrosion shown schematically A Starting condition

B Material thickness disintegration of the part due to uniform corrosion K Grain (crystal)

Contact Corrosion

Contact corrosion occurs when two dissimilar metals are in contact in the presence of an electrolyte. The less precious of the two metals is subjected to the most corrosion, the material loss is uniform. The problem is design related and can be counteracted, e.g. by selection of similar material type combinations.

High Temperature Corrosion

The suitability of materials for use at high temperature is primarily due to the build up of a protective oxide layer on the surface. The presence of this oxide layer reduces the direct contact between the metal and the atmosphere, finally preventing it. The oxida-tion resistance of a material at elevated temperatures depends on the type of oxide which forms. If the oxide is loose and porous, the oxidation process continues until the entire surface is oxidized.

The selection of suitable alloys must be made considering the actual operating condi-tions. The oxidation resistance of Fe-Ni-Cr alloys at isothermal conditions is primarily a function of their chromium content, while the Nickel and Iron components contribute only slightly.

Under cyclic temperature conditions the degree of resistance can change appreciably.

In this case, alloys with a higher Nickel contact are decidedly better, because it reduces the thermal expansion and thereby the flaking off of the oxide.

Fig. 3-29: High temperature corrosion of CrNi-Steel 1.4841 (AISI 314) for use in waste Incineration systems at temperatures approx. 1300 °C (2372 °F) after 5-days-service

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Pitting Corrosion

The pitting corrosion is a localized, pinpoint shaped, penetrating type of corrosion, which in a relative short time can progress through the entire thickness of the metal.

Since it actually eats into the metal and only exhibits point like damage on the surface, it is often difficult to recognize and therefore dangerous. It is greatly accelerated in chlo-ride containing aqueous solutions. The addition of Molybdenum (Mo) and higher chro-mium contents provides better resistance, e.g. 1.4571 (AISI 316) which contains 2.5 % Mo. The material 1.4539 (Uranus B6) with 5 % Mo appreciably improves the resistance compared to 1.4571 (AISI 316).

Fig. 3-30: Pitting corrosion schematically

I Passive layer, with small localized breakthroughs at which pinpoint and hole shaped corrosion occurs

II Active disintegration of the material

Fig. 3-31: Pitting corrosion in a Monel thermowell after usage in a chemical system

Crevice Corrosion

Crevice corrosion occurs due to potentials in the narrow openings caused by the pres-ence of oxygen, such as may exist under a water surface or in narrow gaps, e.g. at the thermowell/flange connection. As a manufacturing countermeasure, the thermowell should be welded to the flange without gaps. The material disintegration occurs as a groove or surface phenomenon. Since crevice corrosion is not always visually evident, it is one of the most dangerous types of corrosion. Steels with higher pitting resistance are also less susceptible to crevice corrosion.

Fig. 3-32: Crevice corrosion schematically

III Passive layer, which will no longer be created in the narrowing gap III Active disintegration of the material

III Surface contamination, deposits, etc.

Intercrystalline Corrosion

Intercrystalline corrosion is caused by selective corrosion. This occurs due the exist-ence of differing potentials at the grain boundaries, or due to nonhomogeneous struc-tures, in which the grain boundaries are dissolved. This type of corrosion occurs prima-rily in stainless steels when exposed to an acidic medium when, due to heating effects (450...850 °C (842...1562 °F)) in austenitic stainless steels and above 900 °C (1652 °F) for ferritic stainless steels the Chromium Carbides precipitate in a combined

“critical“ form at the grain boundaries.

This causes a localized depletion of Chromium in addition to the precipitated Chromium Carbides. For reducing these effects, steels with reduced Carbon content, so called

“Low carbon“ steels such as 1.4404 (316L) or so called stabilized (with Titanium or Nio-bium) steels such as 1.4571 and 1.4550 (AISI 316Ti and 347) are used. The Titanium or Niobium binds with the Carbon to stabilize the Ti- or Nb-carbides, so that even for critical heat effects, the Chromium Carbides cannot be precipitated.

Fig. 3-33: Intercrystalline corrosion schematically

III Passive layer formed at grain boundaries where Chromium has not been depleted III Selective attack near the grain boundaries in zones with depleted Chromium III Grain boundaries with Chromium Carbide

Transcrystalline corrosion

Differing from intercrystalline corrosion the transcrystalline corrosion takes place within the grains in a material structure. It generally occurs along those sliding planes, on which an increased displacement density (the number of displacements which exist which is a measure of previous deformations ) has occurred due to plastic deformations and therefore a higher energy level has resulted. It is a form of corrosion with serious consequences, since it usually becomes apparent only after a breakage has occurred (e.g. after continuous, large tension loads).

Fig. 3-34: Transcrystalline corrosion stress cracks schematically; branching cracks II Passive layer

II Localized penetration through the passive layer

Stress Crack Corrosion

Conditions for the occurrence of stress crack corrosion are the presence of tensile and residual stresses (e.g. caused by welding or cold working), the presence of an electro-lyte and the existence of a crack.

These stresses lead to a movement of the internal displacements in the material. On the surface of the part sliding stages occur. If the surface is covered with a tightly attached blocking oxide layer, it can rupture at the sliding stages and corrosion can attack the material. The interaction between the corrosion and the mechanical loads leads to accelerated crack formation and early failure of the part.

The tendency towards stress crack corrosion is particularly evident in austenitic steels.

This aided by Halogen ion containing corrosion elements, especially ones containing Chlorides of Alkali or Earth Alkali metals, e.g. solutions which contain Sodium, Calcium or Magnesium chlorides. As the chloride ion concentration increases, so does the susceptibility. For this reason in sour gas applications, e.g. according to NACE, a hard-ness of 22 HRC should not be exceeded for steels. Cold worked thermowells should be stress relieved after they have been formed.

Fig. 3-35: Stress crack corrosion as the result of the interaction among of different factors Material

Vibration Crack Corrosion

Vibration crack corrosion is the result of the existence of dynamic tensile stresses in the presence of a corrosive medium. Displacement of the sliding stages of the material on the surface of the part, which occurred due to the vibration forces lead to deep cracks.

Even weak electrolytes can cause an early failure of the part.

Vibration crack corrosion can be counteracted by selecting suitable materials as a func-tion of the attacking medium and by appropriate thermowell design. For critical appli-cations operating near the stress limits, it is essential that design calculations be made.

They should consider especially the critical resonance vibrations (see chapter 3.2.5).

Fig. 3-36: Vibration crack corrosion example of a flange/thermowell connection.

The crack started at the beginning of the threads on the process side.

Stress and vibration corrosion can occur in all metallic materials. The corrosion process for stress crack corrosion is a function of the material and occurs as electrolytic inter-or transcrystalline cinter-orrosion.

Hydrogen Embrittlement

Hydrogen embrittlement is caused by cathodic reactions in an electrolyte. The active hydrogen diffuses into the material and is stored in the tetrahedron and octahedron spaces in the crystal lattice. The crystal lattice is expanded and the hydrogen atoms restrict the elastic movement of the metal atoms (embrittlement). When stressed, cracks are formed eventually leading to failure of the material. As with all crack corro-sion the process remains unnoticed initially and only becomes apparent after a failure has occurred. Special materials are used to prevent this type of corrosion.

The types of damage caused by hydrogen in an aqueous medium in steels are different from those that occur at high temperatures in gases. The damage in gaseous media is based primarily on the decarburization of the steel, while, dependent on the tempera-ture of the material and the pressure in the medium containing the hydrogen, the decarburization may progress from the surface into the inner sections of the steel. The diffusion effects are forced into the background.

In a truer sense, only the damages caused by the inner decarburization are designated as hydrogen attacks. Since the decarburization can be suppressed if the carbon is combined, all carbide building steel alloys are superior to the carbon steels in regard to compressed hydrogen resistance. The resistance increases in general with increasing alloy content.

The specially developed steels for use against compressed hydrogen attack contain above all else, Chromium, Molybdenum and Vanadium elements in low alloy steels such as 1.7362 . They are standardized in SEW 590 (Steel Iron Material Sheet).

In addition to these materials, other steels can be used dependent on the stress con-ditions, particularly the material groups “heat resistant and high heat resistant steel“ as well “stainless and acid resistant steel“.

Selective Corrosion

Differing from the corrosion mechanisms discussed up to this point, selective corrosion only attacks one structure type, while the rest of the structure remains completely intact. For the austenitic CrNi steels it is primarily the Sigma-Phase and the δ-Ferrite which is converted to the Sigma-Phase which is selectively attacked. This type of corrosion occurs predominantly in the welded seams of austenitic CrNi steels. A selec-tive attack occurs for certain mixtures of reducing and oxidizing acids, e.g. hydrofluoric/

nitric acid mixtures and in strong oxidizing sulphuric acid.

General Comments

Even when the material selection is optimized, an aggressive attack could still occur in certain areas, e.g. at welded seams, because during welding, decomposition of the alloy can occur. Partial material compositions may be formed which have a lower resis-tance. In order to prevent this possibility, thermowells manufactured from solid materi-als are used where an aggressive medium is present so that weld seams on the medi-um side are not required. In addition, sometimes two thermowells are using, one placed inside the other.

In general, there are materials suitable for most media, but there is no material that is totally resistant. For the temperature measurements the interaction of aggressive media and high temperatures, disintegration is always a given. The degree depends on the material selection, which may be used to minimize the effects or to maximize the life of the instrument.

For selecting the correct material it is advisable, as a minimum, to use at least the same material quality which was used to make the tank/pipeline. If cost or strength is a con-cern, a material can be used with appropriate properties for the sheath material, e.g.

Glass, Teflon, Tantalum, or an abrasion and corrosion resistant coating such as Stel-lite.