Korrosion in Alkalimetallkreisläufen. II – Untersuchungsmethoden

Corrosion in Alkali Metal Circuits. II – Test Methods The test apparatus for liquid alkali metal corrosion studies of metallic materials are described. The relations between the problems to be studied and the test methods are discussed. Simple and cheap test rigs cause the restriction to stagnant melts and low temperature gradients. However, interesting results can be gained, particularly if the test pots contain instruments. Mock-up tests of technologic problems demand for control and measurement of the most important parameters as flow velocity, temperature gradients, and purity of the melt, if the results should be of value for technical use. The circuits used for this type of experiments can contain up to tons of the alkali metal melts. They may be highly expensive in this case. The experiences gained in the liquid alkali metals experimental techniques may be introduced for corrosion studies in other liquid metals.     

Korrosions- und Kavitationsverhalten von Leichtmetallen in Kühlkreisläufen von Verbrennungskraftmaschinen

Corrosion and cavitation behaviour of light metals in cooling systems of combustion engines Combustion engines are liquid-cooled, in general, in order to transport away dissipated heat and to avoid overheating. Initially, pure water without any further additives was employed for this purpose. With the introduction of high-speed marine diesel engines, however, sometimes after only 20 000 operating hours there occurred damage at the cylinder liners that had been exposed to the cooling water. More than forty years ago, the Research Association Internal Combustion Engines, Registered Association (FVV), Frankfurt am Main, which was founded in the year 1956, commissioned the Institute of Materials Science of the Darmstadt University of Technology to investigate and, if possible, clarify damage of this type and to elaborate corrective action. Since that time, the problem of corrosion in cooling systems has been worked on.     

Zur Korrosion und Rißkorrosion bei Spannstählen

Corrosion and cracking-corrosion of prestressing steels Summarizing report about extent and causes of damages of prestressing steels caused by hydrogen cracking-corrosion. Fundamental research of cracking-corrosion of prestressing steel. Results of cracking-corrosion tests with stressed materials in not pure water, in such moistened sand and of water condensating tests. Hereby consideration of contents of chlorides and sulfates as existing in unfilled tubes; partly little addition of the hydrogen-promotor rhodanide. Result that under practical conditions prestressing steels prefer hydrogen induced cracking-corrosion while hereby especially oxygen-poor electrolytes and local corrosion attacks (pitting scars) tend to this hydrogen reception and therefore to failure. These effects are for example often caused by the conditions of crevice-corrosion and by contact with moistened mineral phases (sands). Such is resulting that sensitive prestressing steels are exposed to the danger of failure already before being embedded with mortar.     

Korrosion und Fügeverfahren

Corrosion and jointing methods In contrast to the mechanical stress analysis of structural parts, the designer is lacking data which take into account the corrosion effect, specially if the parts must be jointed. Examples indicate the type of corrosion damage caused by jointing processes, directly or indirectly due to welding, soldering, metal glueing, metal spraying, or screwing. The causes of such damage, experiments, are described, and remedies are indicated. The survey has provided some knowledge of the problematics of corrosion damage at jointed structural parts.     

Korrosion nichtrostender Stähle und Nickelbasislegierungen in Salpetersäure-Flußsäure-Gemischen

Corrosion of stainless steels and nickel-base alloys in solutions of nitric acid and hydrofluoric acid Reactions involving nitric acid may always result in the contamination of this acid with fluorides. In highly concentrated nitric acid, the presence of small amounts of HF will substantially reduce the corrosion of metallic materials. Mixtures consisting of hydrofluoric acid and hypo-azeotropic nitric acid on the other hand will strongly attack: the metal loss will markedly increase with increasing HNO₃ and HF concentrations as well as with rising temperatures. The investigation covered 12 stainless steel grades and nickel-base alloys. With constant HNO₃ content, corrosion rates will rise linearly when increasing the HF concentration. With constant HF concentration (0.25 M), corrosion rates will increase rapidly with increasing nitric acid concentration (from 0.3 M to 14.8 M). This can best be described by superimposing a linear function and a hyperbolic function that is reflecting the change in the HNO₃ content. Alloys containing as much chromium as possible (up to 46 wt.%) will exhibit the best corrosion resistance. Alloy NiCr30FeMo (Hastelloy alloy G-30) proved to be well suitable in this investigation.     

Ermüdungsverhalten und Korrosion

Fatigue behaviour and corrosion The author studies the influence of corrosion on the strength of materials under simultaneous mechanical load, materials investigated are unalloyed steel, aluminum alloys and chromium steels. The experiments are extended to 10⁷ load cycles. Test media include air, sea-water, 3O% NaCl solution and salt spray. The experiments reveal characteristic influences due to surface treatment (short peened, mechanically polished) and stress frequency. connection with the problem of absolute fatigue resistance it is pointed out, that such a condition can be obtained only under certain conditions and in particular with uniform stress distribution across the whole cross section.     

Methoden der Untersuchung von Reaktormaterialien nach Heißdampf-Korrosion in Versuchskreisläufen und Kernreaktoren

Methods for investigation of reactor materials after corrosion in superheated steam in test circuits and nuclear reactors If it is impossible to make gravimetric determinations one may use planimetric measurements under the optical microscope; the mean thickness of the oxide layer enables the metal losses to be calculated. A combined method involves the descaling of corroded specimens with subsequent determination of the alloy constituent concentration in the surface layer of the descaled metal; this method is suitable in particular when – as is the case during corrosion in steam under heat transfer conditions – part of the oxide is lost to the steam system. The alloy constituents are determined by the microprobe or by X-ray fluorescence. A semi-quantitative method is used for fuel elements; in this case the dimensions of the material are measured prior to use and changes are determined by optical microscopy after use. If the values thus obtained are compared with values determined in out-of-reactor experiments one may even assess the influence of the neutron radiation.     

Kinetische und kristallografische Modellbetrachtungen zur Korrosion und Passivität von Metallen

Kinetic and crystallographic models of metal corrosion and passivity On the basis of detailed studies of short range structures and transformations of structures as well as chemical and electrochemical partial processes in corrosion systems (metal/aqueous electrolyte solution) a model has been investigated. This model covers kinetic as well as crystallographic aspects of the corrosion and passivation of metals from a uniform point of view. Using iron as an example the atomistic actions of metal dissolution and oxide layer formation from the active to passive range will be considered as a result of interactions between structured condensed phases and in comparison with formal kinetic current density-potential correlations.     

Potentialabhängigkeit der Korrosion Mo-freier und Mo-haltiger Stähle in Calciumnitrat-Lösung und Natronlauge

Effect of potential on corrosion of Mo-free and Mo-bearing steels in solutions of calcium nitrate and sodium hydroxide The effect of potential on the corrosion behaviour of three low alloy steels with different carbon and molybdenum contents (0.08 C, 0.01 Mo; 0.08 C, 0.98 Mo; 0.18 C, 1.10 Mo) was investigated in boiling 60 wt.% Ca(NO₃)₂ solution (DIN 50 915) and in boiling concentrated NaOH solutions (20 and 35 wt.% NaOH) by potentiodynamic and chronopotentiostatic polarization measurements (i/E curves) and chronopotentiostatic mass loss measurements (corrosion rate v vs. potential E curves). In Ca(NO₃)₂ solution, i/E measurements give no information about the effect of potential on the anodic dissolution. For the materials investigated, v/E measurements indicate the existence of potential ranges with pronounced differences of the corrosion response. It can be differentiated between active, passive, and transpassive ranges, and also a potential range of secondary passivity was established. Transpassivity and secondary passivity are markedly pronounced with the molybdenum bearing steels but not with the steel free from molybdenum. There are no hints to the occurrence of intergranular attack in the specimens which are free from of internal and external mechanical stresses, whereas such hints could be derived from the shape of the i/E curves. Nevertheless, under mechanical stresses (constant load, CERT conditions) the materials are susceptible to intergranular SCC. The conception that intergranular SCC of low alloy steels in Ca(NO₃)₂ solution is connected with a break-through potential of grain boundary corrosion and hence is to be interpreted as an intergranular attack which, under mechanical stresses, runs in a modified form as SCC with intergranular crack path, cannot be maintained in such general terms. A pronounced active/passive behaviour is observed in NaOH. The effect of potential on anodic metal dissolution which is derived from v/E curve is also established by i/E measurements. For molybdenum bearing steels, the active potential range is somewhat extended to more positive potentials. Manifestations of localized attack, e.g., intergranular corrosion, do not occur. From the investigations, no hints to the cause of the deterioration of the resistance to intergranular SCC in caustic solutions by molybdenum can be derived.     

Erosion und Korrosion of metallischer Werkstoffe in feststoffhaltigen Wässern

Erosion and corrosion of metallic materials in particle containing aqueous solutions The combined effect and mutual influence, respectively, of erosion and corrosion of metallic materials in particle containing flows was investigated. Materials under investigation are carbon steel of different hardness levels, ferritic chromium steel, austenitic stainless steel and brass. Corrosive medium was deionized water, acid (pH 4.5), alkaline (pH 10) and chloride (3% NaCl) solution. Quartz particles were used as abrasive at various particle concentrations, grain sizes and flow velocities. For comparison, tests were performed with pure water and under conditions of idle corrosion. Mechanical or corrosive influences dominate depending on material and test conditions, in some cases, synergistic effects of both mechanisms cause an increased mass loss.     

Einflüsse von Legierungselementen auf die Korrosion und Wasserstoffaufnahme von Eisen in Schwefelsäure – Teil II: Korrosion und Deckschichtbildung

Effects of Alloying Elements on Corrosion and Hydrogen Uptake of Iron in Sulfuric Acid Part II: Corrosion and Formation of Surface Layers The effects of C, S, P, Mn, Si, Cr, Ni, Sn and Cu on the formation of surface layer and hydrogen uptake of iron during corrosion in 1 M H₂SO₄/N₂ were investigated using AES, XPS, SEM and electrochemical permeation techniques. Cu, Sn, P and C are enriched on the surface of iron during corrosion in H₂SO₄. Cu is enriched in the metallic form. P forms a phosphate and phosphide containing surface layer. Ni is not enriched. Cr is preferentially dissolved. Cu, Sn and Ni inhibit the dissolution of iron and thus decrease the hydrogen activity. S, P and Mn (MnS) increase the corrosion and hydrogen activity. Cr forms traps in iron which increase the hydrogen uptake.     

Ursachen und Mechanismen der Spannungsrißkorrosion

Causes and mechanisms of stress corrosion cracking The paper provides a synopsis of the causes and mechanisms of stress corrosion cracking discussed at the present time. The discussion covers: selective metal dissolution (formation of cover layers, influence of lattice imperfections on the anodic metal dissolution, crack spreading rate, anodic hydrogen embrittlement); adsorption and brittle fracture (energy balance, atomistic and electro-chemical aspects); metallurgy (plastic deformation, alloying influence, sliding condition); fracture mechanics (crack formation and growth). Particularly important is the formation of cover layers where stress corrosion cracking depends on a certain ratio of the rate at which the cover layer is reformed and the rate at which surfaces free from cover layer are formed at the bottom of the crack. At high crack expansion rates, the adsorption-brittle fracture mechanism may play a part, as it tends to lower the cohesion at the bottom of the crack. As the crack represents an ideally sharp notch, it is possible to apply the linear elastic fracture mechanics to the crack expansion phenomena.     

Zur Korrosion milchwirtschaftlicher Geräte aus Kunststoffen und ihre Folgeerscheinungen

Corrosion phenomena and their consequences encountered on dairy equipment made of plastics Theorists will be reluctant to apply the term “corrosion” to the interrelations between plastics and food. But the author would like to point out that a food chemist must apply much more exacting scales of value. For instance, the intrinsically unimportant porosity of a tin of condensed milk may, because of the entry of a few milligrammes of iron, render the milk unusuable with coffee. Similarly, a few milligrammes of copper from a cream pump are sufficient for the butter to become rancid within a short time. In the author's opinion, the corrosion of plastics confronts the food chemist with a similar situation. Although the extent of corrosion encountered on plastics will generally, in the food industry be fairly small, the micro-biological and organoleptic consequences are apt to magnify the corrosion effect to such an extent that the foodstuff can no longer be sold.     

Korrosionsverhalten von Plasma- Heißdraht-Auftrag-Schweißungen. Teil I: Beständigkeit von auftraggeschweißten Nickelbasislegierungen gegen Loch- und interkristalline Korrosion

The corrosion properties of Plasma-Hot-Wire-Surfacing Part one: resistance of Nickel-base-alloy claddings against pitting and intergranular attack This paper describes measurements at claddings of nickel-based-alloys C 276, C 4 and C 22 on the St 52-3 steel substrat produced by Plasma-Hot-Wire-Surfacing (PHS). The correct heat control during the welding process is a crucial factor for the preparation of nearly precipitate-free claddings. For this reason it was necessary to optimize all the process data. The claddings itself as well as the rolled reference material (both of nearly the same chemical composition) were examined by means of metallografical and microanalytical techniques. Corrosion measurements show, that the composite materials exhibit high resistance both against pitting and intergranular corrosion and comparable to the properties of the rolled reference material. This was shown by determination of the critical pitting temperature in quinary salt solution, by electrochemical techniques in acidic chloride solution and by investigation in iron-(III)-sulfate-sulphuric acid solution.     

Die Inhibition von Kühlkreisläufen

The inhibition of cooling systems The choice of an efficient corrosion inhibitor has to take in account the type of the cooling system, the materials used as well as the specific operating conditions, for instance

• – very low or very high velocities of the cooling water
• – heat transfer
• – the inhibitor may be asked to have some lubricating properties
• – vibrations, that can cause cavitation or fatigue corrosion
• – interaction between the corrosion inhibitor and other electrochemical systems of corrosion protection (cathodic protection).

In numerous applications it is necessary to improve the inhibition by an addition of other effective substances (like dispersants, hardness stabilizers, mixture of inhibitors with selective protection properties).     

Zu einem Phänomen der Lochkorrosionsbegrenzung bei hochlegierten Sonderedelstählen und NiCrMo-Legierungen in Chloridlösungen

On a phenomenon of the limitation of pitting corrosion at high alloyed special stainless steels and NiCrMo-alloys in chloride solutions Testing the pitting corrosion resistance of high alloyed special stainless steels and NiCrMo-alloys in chloride solutions there was observed a limitation of the pitting corrosion range toward more positive potentials. Above this limitation, the so-called pitting corrosion limitation potential, the pit initiation by all means is prevented, but the growth of pits which had been initiated before in the pitting corrosion range not necessarily comes to a stop. Therefore current density-potential curves which are obtained by downward polarization after an initial potential jump into the transpassive region and chronopotentiostatic tests are more suitable to investigate this phenomenon than cyclic polarization measurements and potentiostatic alteration tests. There is indicated a dependence of this phenomenon on temperature. The phenomenon of a limitation of the pitting corrosion range toward more positive potentials has been found until now at alloy 926, alloy 31, alloy 28, alloy 59 and alloy C-276.     

Korrosion und Korrosionsinhibierung von Aluminiumpigmenten im alkalisch wäßrigen Medium

Corrosion and inhibition of corrosion of aluminium pigments in alkaline aqueous medium Flakelike aluminum pigments (Al-content > 99.5%) were corroded in an aqueous alkaline mixture of water and butyl glycol in the ratio 9: 1. Chelating agents like citric acid or polyacrylic acid inhibit this corrosion reaction. The temporal progress of the corrosion reaction can be determined by volumetric analysis of the evolved hydrogen and furthermore, with addition of the corrosion inhibitors, by measuring the electrical conductivity of the solution. The corrosion reaction consists essentially of two steps:

• 1 In the latency period no or only little corrosion takes place; during this, time the protective layers (normal oxide layer or layers reinforced by reaction products of inhibitor and aluminum) were dissolved.
• 2 After that the actual corrosion reaction takes place.

It was proved that changes of the pH-value or addition of corrosion inhibitors only influence the duration of the latency period; after that period the rate of the corrosion reaction is nearly independent from the examined conditions.     

Korrosion in der Wärmeeinflußzone geschweißter chemisch beständiger Stähle und Legierungen und ihre Verhütung

Corrosion in the heat-affected zone of welds in chemically resistant steels and alloys, and respective preventive measures As to weldability, chemically resistant steels and alloys can be classified as follows: (1) weldable without any restriction; (2) weldable only with reduced wall thickness; (3) weldable only with ultimate thermal treatment. These restrictions are due to the precipitation of chromium carbide and intermetallic phases at the grain boundaries; this effect gives rise to a Cr and Mo depletion and, finally, to intercrystalline corrosion susceptibility. In view of the fact that weldability requires the delay of the precipitation of chromium carbides at 650°C for at least 1 hour, and of intermetallic phases at 900°C for at least 10 min it is possible to reduce the carbon content and/or to add stabilizing elements (Ti, Nb). It should be taken into account, however, that the precipitation behaviour is not a function of carbon concentration, but rather of carbon activity which, again, depends from the overall composition of the respective alloy. This activity is increased by Ni and Si, while Mn and N function as decelerators and, consequently, contribute to weldability. In the case of the steel X 3 CrNiMoN 17 13 5 the addition of N inhibits not only the carbide precipitation but also the precipitation of the Chi-phase (at 950°C). In the case of the alloy NiMo 16 Cr the gradual reduction of the contents of secondary constituents has resulted in a practically pure ternary system characterized by high precipitation resistance: carbides are precipitated only after 5 hours at 800°C.     

Korrosion unter Wärmedämmstoffen

Corrosion in contact with thermal insulations Again and again there are corrosion failures on metallic pipes or tanks coated with thermal insulations. Like in other corrosion cases they are caused by superposition of several influences. The importance of these factors is discussed generally and with the aid of special examples.     

Zum Einfluß von Chloracetylchloriden und Chloressigsäuren auf die Korrosion metallischer Werkstoffe in Trichlorethylen und Perchlorethylen

The influence of chloroacetyl chlorides and chloroacetic acids on the corrosion of metals in trichloroethylene and tetrachloroethylene The corrosive behaviour of selected stainless steels, nickel, copper and zinc materials in trichloroethylene and tetrachloroethylene containing dichloroacetic acid and dichloroacetyl chloride or trichloroacetic acid and trichloroacetyl chloride in concentrations between 100 and 10000 ppm from room temperature to the boiling point was investigated. NiCu 30 Fe was resistant in all systems investigated. Only tetrachloroethylene containing trichloroacetyl chloride corroded the stainless steels. Corrosion rates up to 0.07 mm/y and a tendency towards pitting corrosion were detected. Trichloroethylene containing dichloroacetic acid proofed aggressive towards Cu-DHP, CuZn 15, and CuSn 6. Corrosion rates between 0.14 and 0.32 mm/y were observed. Tetrachloroethylene containing trichloroacetic acid was far less corrosive towards copper materials. In tetrachloroethylene, containing trichloroacetyl chloride, of the copper materials investigated, CuZn 15 and CuAl8 were relatively strongly attacked (corrosion rate 0.10 mm/y). Of the investigated materials, Zn 99.95 was the least resistant to corrosion. Solutions containing acids and acid chlorides proofed to be the most aggressive with corrosion rates between 0.56 and 0.66 mm/y. Some corrosion products, developed on the metal surfaces, were analysed.