Korrosionsreaktiorren von elementarem Schwefel mit unlegiertem Stahl in wäßrigen Medien

Abstract

Unlegierter Stahl wird durch elementaren Schwefel in Gegenwart waariger Medien bei Raumtemperatur verhaltnismafiig stark angegnffen. Die Korrosion erfolgt bevorzugt an den Beruhrungsstellen der Feststoffe Fe und S unter Muldenbildung. Gleichzeitig erfolgen eine geringe pH-Absenkung sowie eine geringfugige Bildung von H2S und Sulfat-Ionen. Der Vorgang wird durch Neutralsalze sehr stark stimuliert, durch Phosphate inhibiert und durch alkalische Medien, wie z. B. Na,CO, und Ethylamin rnit pH > 12, vollig unterbunden. Dies kann zum Korrosionsschutz genutzt werden. Mit zunehmender Salzkonzentration (c > 1 mom) nehmen die Abtragungsraten wieder ab. Durch zunehmende Stromungsgeschwindigkeit konnen die Abtragungsraten zunehmen, wobei aber nur gleichformiger Flachenangriff auftritt. Elektrochemische Untersuchungen zeigen, dafi die Reduktion des Schwefels am Korrosionsprodukt FeS stattfindet und fiir die Korrosion geschwindigkeitsbestimmend ist. An Pt-Kathoden wird bei Abwesenheit von FeS keine Schwefelreduktion beobachtet. Als Startreaktion fir die Korrosion im System Fe/S/H20 wird eine geringe Disproportionierung des S zu H2S04 angenommen, wobei FeS entsteht.Plain carbon steels are rather severely attacked by elemental sulphur at room temperature in the presence of aqueous media. The corrosion occurs preferentially at the places where the two solid substances iron and sulphur are in contact with each other and results in shallow pit formation. At the same time the pH is also decreased slightly and small amounts of H,S and sulphate ions are formed. Neutral salts stimulate the corrosion process whereas phosphates inhibit it and the alkaline media such as Na2C03 and ethylamine with pH > 12 prevent it completely. The latter can be made use of for corrosion protection. At high salt concentrations (c > 1 moV1) the corrosion rate, however, decreases with increasing salt concentration. The corrosion rate may increase with increasing flow velocity of the medium, but the corrosion takes place uniformly.The results of electrochemical investigations show that the reduction of sulphur occurs at the corrosion product FeS and is the rate controlling step. No sulphur reduction is observed on platinum electrodes when no FeS is present. It is assumed that the starting reaction to initiate corrosion in the system Fe/S/H,O is a slight disproportionation of S to H,S04 resulting in the formation of FeS.

Metabolites of an Oil Field Sulfide-Oxidizing, Nitrate-Reducing Sulfurimonas sp. Cause Severe Corrosion

“…5, reaction IV), as evidenced by higher corrosion rates measured with FeS-covered steel surfaces (Fig. 4) and as seen elsewhere in experiments with chemically prepared S 0 (24, 25). The cathodic reduction observed here was likely facilitated by reactive –SH groups present at FeS surfaces (51, 52).…”

“…Carbon steel pipelines carrying sulfidic produced waters or harboring sulfate-reducing bacteria will likely develop iron sulfides, such as mackinawite (FeS), as a corrosion product on internal steel surfaces (41–43). Previous reports attributed a catalytic role for FeS in corrosion mechanisms involving chemically prepared S 0 (24, 25). Here, we investigated the effects of FeS in abiotic incubations with biologically generated (biogenic) S 0 , as well as with nitrite.…”

“…Laboratory studies have similarly detected increased corrosion in experiments with mixed microbial communities and nitrate addition (21–23). The formation of zero-valent sulfur (S 0 ), an intermediate of sulfide-oxidizing, nitrate-reducing bacteria (soNRB), has been hypothesized to be responsible for such corrosion (19, 21, 23), given that S 0 is known as a strong oxidant of iron (24–27). Other mechanistic explanations of MIC under nitrate-reducing conditions have also been offered, including the formation of nitrite, consumption of cathodic hydrogen, direct electron uptake from Fe 0 , and sulfur disproportionation (18–23).…”

Localized corrosion of lean duplex stainless steels in H2S‐containing wet atmosphere from urban wastewater treatment units

“…From these results, the formation of FeS or elemental S in presence of H 2 S is very probable, [34] even if X-ray diffraction analyses would be needed to confirm these results. Elemental S is known to have significant detrimental effect on the corrosion resistance of carbon steel, [35][36][37] but also on corrosion-resistant alloys, [38][39][40][41] with possible modification of passive film stability. The detrimental effects of elemental sulfur are furthermore enhanced in aerated environments and in the presence of high chloride contents, particularly for Ni-based alloys but the same effect on stainless steel can be hypothesized.…”

Formation of Ferrous Sulfide Film from Sulfite on Steel Under Anaerobic Conditions

“…26 The film, which probably contains polysulfide, has a corrosion potential of -0.66 V SCE under neutral conditions and is reduced when the potential is lowered.…”