输油管道防腐
输油管道防腐
随着国民经济的发展,管道输油的优点日益突显出来。输油管道基本上都采用碳素钢无缝钢管、直缝电阻焊钢管和螺旋焊缝钢管。输油管道的敷设一般采用地上架空或埋地两种方式。但无论采用那种方式,当金属管道和周围介质接触时,由于发生化学作用或电化学作用而引起其表面锈蚀。这种现象是十分普遍的。金属管道遭到腐蚀后,在外形、色泽以及机械性能方面都将发生变化,影响所输油品的质量,缩短输油管道的使用寿命,严重可能造成泄漏污染环境,甚至不能使用。由于金属腐蚀而引起的损失是很大的,因此,了解腐蚀发生的原因,采取有效的防护措施,有着十分重大的意义。根据金属腐蚀过程的不同点,可以分为化学腐蚀和电化学腐蚀两种。
1. 化学腐蚀
单纯由化学作用而引起的腐蚀叫化学腐蚀。例如,金属裸露在空气中,与空气中的O2 、H2S 、 SO2、 CI2等接触时,在金属表面上生成相应的化合物(如氧化物、硫化物、氯化物等)。通常金属在常温和干燥的空气里并不腐蚀,单在高温下就容易被氧化,生成一层氧化皮(由FeO 、Fe2O3、Fe3O4组成),同时还会发生脱碳现象。此外,在油品中含有多种形式的有机硫化物,环烷酸它们对金属输油管道也会产生化学腐蚀。
2. 电化学腐蚀
当金属和电解质溶液接触时,由电化学作用而引起的腐蚀叫做电化学腐蚀。它和化学腐蚀不同, 是由于形成了原电池而引起的。金属管道与含有水分的大气,土壤、湖泊、海洋接触。这些介质中含有CO2、SO2、HCI 、NaCI 及灰尘都是不同浓度的电解质溶液,金属本身由于含有杂质,由于铁元素和杂质元素的
电位不同,所以当钢铁暴露于潮湿空气中时,由于表面的吸附作用,就使铁表面上覆盖一层极薄的水膜。水的电离度虽小,但仍能电离成H+离子和OH –离子,在酸性介质的大气环境中H+的数量由于水中溶解了CO2、SO2等气体而增加。因此,铁和杂质就好像放在含有H+、OH –、HCO3、HSO3-等离子的溶液中一样,形成了原电池。铁为阳极,杂质为阴极。
由于铁和杂质紧密地接触,电化学腐蚀作用得以不断进行。铁变成铁离子进入水膜,同时多余的电子移向杂质。水膜中的Fe2+离子和OH-离子结合,生成Fe(OH)2附着在铁表面,这样铁便很快遭受腐蚀。其反应如下:
阳极(铁) Fe=Fe2++2e
Fe2++2OH-=Fe(OH)2
然后,Fe(OH)2被空气中的氧气氧化为Fe(OH)3。Fe(OH)3及其脱水产物Fe2O3是红褐色铁锈的主要成分。该腐蚀实际上是在酸性较强的情况下进行的。 在一般情况下,如果铁表面吸附的水膜酸性很弱或是中性溶液,则在阳极也是铁氧化成Fe2+离子,在阴极主要是溶解于水膜中的氧得到电子:
阳极 2Fe=2Fe2++4e
阴极 O2+2H2O+4e=4OH-
所以介质中不仅H+离子能引起金属腐蚀,含有氧时也能腐蚀。
3. 腐蚀的防止
地下管道的腐蚀主要有电化学腐蚀、杂散电流腐蚀和微生物的腐蚀等。影响金属腐蚀的因素包括金属的本性和外界介质两个方面。就金属本身来说,金属越活泼就越容易失去电子而被腐蚀。外界介质对金属腐蚀的影响也很大,如果金属在潮湿的空气中,接触腐蚀性气体或电解质溶液,都易于腐蚀。输油管道的防腐
一般采用如下方法:
3.1上管道外防腐
根据以往经验,普遍认好以红丹油性防锈漆、红丹醇酸防锈漆等作底漆。这些漆防绣性能好,与钢铁表面附着力强。施工现场用樟丹和清油现配,要掌握好比例:一般按下面比例配制:樟丹56.6%、清油37.8%。另外,加汽油或煤油
5.6%左右,以利调和快干。待底漆干燥后,均匀涂刷两遍面漆。面漆材料有很多种,但使用较多的为铝粉(或称银粉)漆。铝粉漆漆膜平滑、坚韧、附着力强,并有金属光泽。施工现场配制时,其配比为:铝粉:清油或清漆:溶剂汽油(重量比为1:1.5:1.5)。
3.2埋地管道的防腐绝缘
3.2.1内防腐
由于油品的洁净度不同,油品中仍残留一些杂质,水分、微生物,前面也提到过,因为这些残留物的存在,管道内壁也会形成原电池,造成腐蚀,产生的锈片将严重影响油品质量。一般内防腐采用036耐油防腐涂料。该涂料化学稳定性好,机械性能高,不污染油品,使用方便。施工中要求对底材处理,用喷丸除锈,质量应达到国标Sa2.5级(GB8923-88)。做两道036-1底漆,再涂两道036-2面漆。按规定严格控制涂漆厚度。
3.2.2外防腐
埋地管道的防腐绝缘,一般分三级;当土壤电阻率
4. 埋地管道的电化学保护
除了上述涂料防腐保护外,还可以配合牺牲阳极保护。在电化学腐蚀中,金属电位高的为阴极,电位低的为阳极,电流从高电位到低电位,低电位金属失去电子而被氧化,产生腐蚀。如找出另外一些金属的电位比管道低,这些电位较低的金属失去电子而被腐蚀。保护了埋地管道,这种做法叫牺牲阳极保护。一般采用镁阳极和锌阳极。具体做法按着《埋地刚质管道牺牲阳极阴极保护设计规范》执行。
Pipeline Corrosion
With the development of the national economy, the increasingly highlighted the advantages of oil pipeline. Oil pipeline are basically using carbon steel seamless steel pipe, straight seam resistance welding steel pipe and spiral welded steel pipe Oil pipeline laying on the ground of general use overhead or buried in two ways. But either way, when the metal pipe and the surrounding medium contact, due to the occurrence of chemical or electrochemical action and cause the surface corrosion. This phenomenon is very common. Metal pipe by corrosion in appearance, color, and mechanical properties will be changed, affect the transmission oil quality, shorten the pipeline service life, serious may result in leakage and environmental pollution, can't even use As a result of metal corrosion caused by loss is big, therefore, to understand the corrosion causes, take effective measures, it is of great significance.
According to the corrosion of metals in different points, can be divided into two kinds of chemical and electrochemical corrosion
1. Chemical Corrosion
Pure corrosion caused by chemical action is called a chemical corrosion. For example, the metal is exposed to the air, and air in the O2, H2S, SO2, CI2, contact, the surface of the metal to generate the corresponding compounds (such as oxide, sulfide, chloride). Usually metal at room temperature and dry air is not corrosive, single at high temperature is easy to be oxidized, generate a layer of oxide (FeO, Fe2O3, Fe3O4 by composition), while also occurs decarburization. In addition, the oil contains a variety of forms of organic sulfides, naphthenic acid on metal pipeline will produce chemical corrosion.
2. Electrochemical Corrosion
When the metal and electrolyte solution contact, corrosion caused by electrochemical action called electrochemical corrosion. It and chemical corrosion, is due to the formation of primary cells caused by. The metal pipe and the water containing atmosphere, soil, lakes, ocean contact. The medium containing CO2, SO2, HCI, NaCI and dust are different concentration of electrolyte solution, the metal itself due to impurities, due to iron and impurity elements at different potentials, so when the steel is exposed to moist air, due to surface adsorption, makes the iron surface was covered with a thin water film. Water ionization degree is small, but still can be ionized into H+ + and OH - ions in acidic medium, the atmospheric environment in H+ number due to the
dissolved CO2, SO2 gas and increase. Therefore, iron and impurity as if placed in the containing H+, OH -, HCO3, HSO3- plasma solution, forming a galvanic cell.Iron as anode, impurities as cathode.
Because iron and impurity in close contact, electrochemical corrosion is ongoing. Iron into iron ions into the water film, at the same time the excess electrons move to the impurity. Water film in the Fe2+ and OH- ion binding, generating Fe (OH) 2 attached to the surface of iron, the iron will soon suffer corrosion. The reaction is as follows:
Anode (iron) Fe=Fe2++2e
Fe2++2OH-=Fe (OH) 2
Then, Fe (OH) 2 is oxidized by oxygen in air for Fe (OH) 3.Fe (OH) 3 and the dehydrated product Fe2O3 is the main component of red brown rust.The corrosion is actually in strong acid condition.
In general, if iron is absorbed on the surface of the water film acid a weak or neutral solution, the anode is iron oxide into Fe2+ ion, the cathode is dissolved in the water film in oxygen to obtain electronic:
Anode 2Fe=2Fe2++4e
Cathode O2+2H2O+4e=4OH-
So the medium not only H+ ions can cause corrosion of metal, containing oxygen can also corrosion.
3. Corrosion Prevention
Underground pipeline corrosion mainly electrochemical corrosion, stray
current corrosion and microbial corrosion. Effect of metal corrosion factors include metal nature and the external medium two aspects. On the metal itself, metal and more active more easily lose electrons and corrosion.The external medium effect on metal corrosion is very big also, if the metal in the moist air, exposure to a corrosive gas or electrolyte solution, easy corrosion. Pipeline anticorrosion generally use the following methods:
3.1 Pipeline Anticorrosion
According to past experience, generally identified with red lead oil Hongdan anti-rust paint, alkyd antirust paint, primer.These paint antirust performance is good, strong adhesion with the steel surface.Construction site with minium and oil mix, have a good grasp of scale: according to the following proportions: Zhang Dan 56.6%, oil 37.8%.In addition, gasoline or kerosene 5.6%, Eli and quick-drying. For primer drying after two times, even brushing paint.Finish material there are many, but is used more for aluminum powder paint (or silver).Aluminum paint smooth, tough, strong adhesion, and metallic luster.The construction site preparation, composition: Powder: oil or varnish: solvent gasoline (weight ratio of 1:1.5:1.5).
3.2 Buried Pipeline Anticorrosion Insulation
3.2.1 Internal Corrosion
As a result of oil cleanliness, oil still some residual impurities, water, microorganism, previously mentioned, because the existence of these residues, inner wall of the pipeline will form a galvanic cell, causing corrosion,
resulting with flakes of rust will seriously affect the quality of oil. General corrosion using 036 anticorrosive coating.The coating of good chemical stability, high mechanical property, no pollution to oil, easy to use.Construction requirements for substrate processing, using shot blasting, quality should meet the national standard Sa2.5 (GB8923-88).Do two 036-1 primer, two 036-2 topcoat coating.According to the regulations strictly control the coating thickness.
3.2.2 External Coating
Buried pipeline anticorrosion insulation, generally divided into three grade; when the soil resistivity
4.the Electrochemical Protection of Buried Pipeline
In addition to the above coating corrosion protection, but also with the sacrificial anode protection.On the electrochemical corrosion of metal, high potential for cathode, low potential for anode, current from a high voltage to the low potential, low potential metal lost electrons is oxidized, producing corrosion. Find some other metal potential than pipeline low, these potential lower metal lost electrons being eroded.Protection of buried pipeline, this approach is
called the sacrificial anode protection. Generally use the magnesium anode and zinc anode.Specific practices by just "buried pipeline cathodic protection design code execution".