金属在某些环境中的腐蚀课件

1、金属在某些环境中的腐蚀课件金属在某些环境中的腐蚀课件第三节 金属在土壤中的腐蚀特点Corrosion in Soils 2第三节 金属在土壤中的腐蚀特点43.3.1 IntroductionSoil corrosion is a complex phenomenon, with a multitude of variables involved. Chemical reactions involving almost each of the existing elements are known to take place in soils, many of which are not yet

2、 fully understood. 33.3.1 IntroductionSoil corrosiVariations in soil properties and characteristics across three dimensions can have a major impact on corrosion of buried structures.4Variations in soil properties Various types of soil differ considerably in structure and composition, and therefore a

3、lso in corrosivity to materials. Also climatic influences (precipitation, temperature) and their fluctuations, air movements (erosion, aeration), and sunlight may very much influence the properties of the soil and therefore the corrosion behavior of materials in soil. 5Various types of soil differ c

4、Some general rules: Soils with high moisture content, high electrical conductivity, high acidity, and high dissolved salts will be most corrosive. The effect of aeration(通风) on soils is somewhat different from the effect of aeration in water because poorly aerated conditions in soils can lead to acc

5、elerated attack by sulfate-reducing(硫酸盐还原) anaerobic bacteria(厌氧细菌). 6Some general rules: Soils with3.3.2 Soil Variables a.Water Water(liquid) - the essential electrolyte saturated / unsaturated water flow unsaturated - represents movement of water from wet areas towards dry soil areas. Saturated wa

6、ter flow is dependent on pore(孔) size and distribution, texture, structure, and organic matter. 73.3.2 Soil Variables a.Water9b.Degree of aerationThe oxygen concentration decreases with increasing depth of soil. In neutral or alkaline soils- important effect on corrosion rate - participation in the

7、cathodic reaction. However, certain microbes (such as sulfate reducing bacteria) corrosion rates can be very high, even under anaerobic conditions. 8b.Degree of aerationThe oxygenc.pHSoils usually have a pH range of 5-8. More acidic soils obviously represent a serious corrosion risk to common constr

8、uction materials such as steel, cast iron and zinc coatings. 9c.pHSoils usually have a pH rad.ResistivityResistivity has historically been used as a broad indicator of soil corrosivity. Since ionic current flow is associated with soil corrosion reactions, high soil resistivity will arguable slow dow

9、n corrosion reactions. 10d.ResistivityResistivity has h土壤腐蚀性划分标准极强强中弱级弱土壤电阻率(m) 100土壤含盐量(%) 0.75 0.75-0.1 0.1-0.05 0.05-0.01 40 3土壤 pH 值电解失重(g/24hr) 6 4.5-5.5 6-3 5.5-7 3-2 7-8.5 2-1 8.5 50 20-50 35 20-30 50 20-50 10-20 7-10 100 20-100 10-20 5-10 60 45-60 20-45 100 50-100 20-50 2011土壤腐蚀性划分标准极强强中弱级弱土

10、壤电阻率(me.Redox potentialThe redox potential essentially is a measure of the degree of aeration in a soil. An high redox potential indicates a high oxygen level. Low redox values may provide an indication that conditions are conducive to anaerobic microbiological activity. 12e.Redox potentialThe redox

11、 potf.Chloride levelChloride ions are generally harmful, as they participate directly in anodic dissolution reactions of metals and their presence tends to decrease the soil resistivity. 13f.Chloride levelChloride ions h.Sulfate levelSulfates are generally considered to be more benign in their corro

12、sive action towards metallic materials. However, concrete may be attacked as a result of high sulfate levels. The presence of sulfates does pose a major risk for metallic materials in the sense that sulfates can be converted to highly corrosive sulfides by anaerobic sulfate reducing bacteria. 14h.Su

13、lfate levelSulfates are ge3.3.3 Forms of Corrosion in SoilsSmall metalwork- general corrosionEquipments in large size/ long pipelines-localized corrosion153.3.3 Forms of Corrosion in SoA.Corrosion by Concentration Cellsthe extent of aeration: differentially aerated soils; proportion immersed in wate

14、r; coverage by moist porous construction materials; biological growth and deposits; the composition and aggressivity of the soil (pH, salt content, resistivity, etc.). 16A.Corrosion by Concentration CAnodic regions are produced under the following conditions: no aerations, high salt content, low pH

15、value, high moisture.Cathodic regions are produced under the following conditions: good aeration, low salt content, higher pH value, moderately wet areas.17Anodic regions are produced unIn the case of corrosion caused by concentration cells, anodic regions have a more negative potential than cathodi

16、c regions. The potential difference may be up to 300 mV.The solution for these problems is cathodic protection.18In the case of corrosion cause土壤腐蚀粘土 (贫氧区)砂土(富氧区)阴极阳极埋地钢管地面 构成氧浓差电池的一种情况Aeration induced corrosionDifferential-aeration cells can be set up between the top and bottom surfaces of pipes, o

17、r down buried piles and well casings owing to the generally lower oxygen content at deeper soil depths. The same effect can happen where metal systems pass under roads, which are relatively impervious to oxygen.19土壤腐蚀粘土 (贫氧区)砂土(富氧区)阴极阳极埋地地面 构成B.Microbiologically influenced corrosion (MIC)Microbiolog

18、ically influenced corrosion (MIC) refers to corrosion that is influenced by the presence and activities of microorganisms(微生物) and/or their metabolites (the products produced in their metabolism(新陈代谢). /tekbrefs/sib96148/ 20B.Microbiologically influencedThe forms of Microbiologically Influenced Corr

19、osion are not unique. Biocorrosion results in pitting, crevice corrosion, selective dealloying, stress corrosion cracking, and under-deposit corrosion. 21The forms of MicrobiologicallyTubercles 节瘤 Term refers to a mound隆起 on the metal surface which contain biological species, probably in an adherent

20、, slimy粘性的 material. Eventually the mounds contain corrosion products resulting from their electrochemical activity.22Tubercles 节瘤 probably in an adThiobacillus-硫杆菌Gallionclla-铁氧化菌23Thiobacillus-硫杆菌25Tuberculation can result from non biomass materials e.g., carbonates, silicates, phosphates, greases

21、(油脂), mud, road debris(碎片). The result is pitting or even crevice corrosion underneath.24Tuberculation can result from Microbiologically influenced corrosion in a 316 stainless steel weld /types_of_corrosion/microbiologically_influenced_biological_microbial_corrosion.htm Iron-oxidizing bacteria can perforate a 5mm thick 316 stainless steel tank in just over a month! 25Microbiologically influenced cPrevention of MICRegular mechanical cleaning if possible .Chemical treatment with biocides to control the population of bacteria. Complete drainage and dry-stor