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The corrosion of above-ground pipelines is caused by the combined action of corrosive ions (Cl-, S2-), CO2, bacteria and dissolved oxygen. Dissolved oxygen is a strong oxidant, it is easy to oxidize iron ions to form precipitation, and the relationship between dissolved oxygen and corrosion rate is linear. Sulfate-reducing bacteria will the existence of the sulfate-reducing hydrogen sulphide in the water, may lead to pipe hydrogen induced cracking and stress corrosion cracking, corrosion products generated ferrous sulfide and adhere on the surface of the steel is poor, easy to fall off, is potential, as the cathode constitute an active micro battery and steel matrix, and continue to produce corrosion to the steel substrate. Saprophytic bacteria adhere to the pipeline and cause fouling blockage, and also produce oxygen concentration cells and cause pipeline corrosion. The oil-water mixture in the surface pipeline may enter the sewage tank after separation. Therefore, when choosing anti-corrosion measures for the above-ground pipelines in the oil fields, the protection effect, construction difficulty, cost and other factors should be considered. Some commonly used anti-corrosion measures are for oil field above-ground pipelines:

 

Coating

There are many anticorrosive coatings on pipelines, and their performance is different. Choosing appropriate coatings can greatly extend the service life of pipelines. According to the corrosive environment, transport media and other conditions to choose the appropriate coating. The outer protective coating is the first and most important barrier of the above-ground steel pipe, mainly organic coating and metal coating (or coating). Organic coatings can be divided into epoxy resin, modified phenolic epoxy, asphalt, coal tar and other coatings. The experimental results show that the surface of the coating does not bubble when soaked in brine and oil, and the coating meets the requirements of API RP 5L2 adhesion and peel test, indicating that the coating has good adhesion. The coating is heated at 250℃ for 30min and then cooled by water at room temperature. The coating surface has no peeling, no cracking, no bubble, no adhesion loss, etc., that is, the coating has good heat resistance. According to ASTM D522, ASTM D968 and other standards to carry out bending and wear tests, the coating also has good bending and wear resistance.

 

Cathodic protection

It is not easy to coat the internal surface for small diameter pipelines (pipe diameter less than 60mm), even if the coating is completed indoors, it is difficult to achieve 100% pinhole free. In addition, the inner wall coating is often subjected to wear in the process of use, so the use of cathodic protection can effectively reduce corrosion perforation. Sacrificial anode protection is the earliest cathodic protection method, which is simple to operate and does not require power supply. The sacrificial anode materials commonly used in China include magnesium, zinc, aluminum and their alloys.

The output current of the sacrificial anode depends on its shape and size. In the laboratory test of magnesium, zinc, an aluminum alloy of cathodic protection potential (relative to the copper/copper sulfate reference electrode), three types of alloy are accord with the requirement of oil and gas station cathodic protection specification (cathodic protective potential is 0.85 V or more), including aluminum alloy anode protective effect is best, magnesium anode and zinc alloy anode is poorer.

 

Special joint

The special joint is designed to solve the damage to the interface coating caused by pipe welding after coating. Methods include: using refractory insulation material and high-temperature coating; Or use a new type of high temperature heat insulation ceramic joint, which has good heat insulation performance and corrosion resistance, as well as in the temperature of drastic changes in the performance of the burst and permeability resistance, but the disadvantage is that the strength and toughness is poor. Laboratory tests show that under the conditions of drastic changes in temperature, the crack resistance and penetration resistance of the joint can meet the requirements. However, under the premise of ensuring the strength and toughness, the joint wall thickness is too thick, and the change of inner diameter will affect the normal construction of the pipeline. The use of refractory insulation materials and high-temperature coating joints can fully meet the requirements of use.

 

When talking about the heat treatment of austenitic U-shaped stainless steel tubes, most people think it’s not necessary because of sensitization and high solution treatment temperature, it is easy to cause deformation of the pipe. In fact, the heat treatment of Austenitic stainless steel is inevitable, heat treatment can not change the structure of stainless steel tubes, but can change the processability.

For example, due to low carbon content, 304 stainless steel heat exchange tube is difficult when normalizing to make the surface roughness of the gear shaping cutter to meet the requirements, reduce the tool life. The low carbon martensite and iron cable structure obtained after incomplete quenching can greatly improve the hardness and surface roughness, and the service life of the pipe can also be increased by 3 ~ 4 times. In addition, the u-shaped heat exchange tube bending part has a small bending radius and obvious work hardening phenomenon, heat treatment is necessary, and compared with the whole equipment for heat treatment, austenitic stainless steel pipe solution heat treatment, pickling passivation is much simpler. In this paper, a series of tests have been taken on U-shaped tubes with different specifications, bending radius and heat treatment conditions, and the necessity of heat treatment for U-shaped tubes made of austenitic stainless steel has been analyzed.

 

Experimental materials:

304 stainless steel U-tube

Size: 19*2mm, bending radius: 40, 15, 190, 265, 340mm

Size: 25*2.5mm Bending radius: 40, 115, 190, 265, 340,mm

Heat treatment: untreated, subsolid solution treatment, solid solution treatment

 

Hardness Testing

The bending section of u-shaped heat exchange tube without heat treatment and subsolid solution treatment: with the decrease of bending radius, the hardness value increases. The hardness value of heat exchange tube after solution treatment (compared with that before bending) has no obvious change. This indicates that Austenitic stainless steel work hardening effect is obvious, and with the increase of deformation, the trend of work hardening increases.

 

Microscopic inspection

For the u-shaped bend section with a bending radius of 40mm: there are a lot of martensite and slip lines in the microstructure without heat treatment, and the equiaxed shape of austenite in the microstructure has completely disappeared (too much martensite will make the steel brittle). Most of the martensite in the subsolid solution treated tissue has been transformed, but a small amount of martensite still exists.

After solution treatment, the austenite grains were equiaxed and no martensite was found. The slip bands and martensite also existed in the unheated microstructure of u-shaped tubes with bending radius R of 115, 190, 265 and 340mm after bending, but the content decreased gradually with the increase of bending radius. When the bending radius R of the U-shaped tube is greater than or equal to 265mm, the effect on the microstructure before and after heat treatment is not significant. When the bending radius R is less than 265mm, there is martensite in the microstructure of unheated U-shaped tubes, and the content of martensite decreases with the increase of heat treatment temperature (subsolid solution treatment and solid solution treatment).

 

Intergranular corrosion test

By microscopic examination, it was found that the presence of martensite did not affect intergranular corrosion. Although there is a large amount of martensite in the absolutized microstructure, there is no tendency of intergranular corrosion along with the distribution of martensite. Some grain boundaries widened before and after solution treatment, and the distribution of grain boundaries widened was independent of the distribution of martensite. On the basis of microscopic examination after the corrosion test, the bending test was carried out for u-shaped tubes in various states according to the test standard. No intergranular corrosion cracks were found in the tubes after bending 180°.

 

Solution treatment temperature

The effect of solution treatment is affected by the low solution temperature, and the results of microstructure and hardness can not be obtained. If the temperature is slightly higher, defects such as concave or crack may appear inside the U-shaped segment.

 

From the experiment, it is known that the martensite transformation of stainless steel after cold processing, the influence of corrosion resistance is far greater than the stress. When the bending radius of the u-shaped tube is less than 115mm, the microstructure of the u-shaped tube before and after solution treatment is significantly different. For this small radius U-shaped pipe bend segment, solid solution treatment should be performed after cold forming. If there is no requirement for higher intergranular corrosion resistance, it is recommended that the u-shaped bending section with a bending radius less than or equal to 265mm be treated with solution treatment (note to eliminate residual stress). For u-shaped heat exchange tubes with large radius curvature, the bending section may not be treated with solution, except for stress corrosion sensitive environments. Because the small pipe diameter fluid resistance is large, it is inconvenient to clean and easy to block the structure, and the large diameter stainless steel pipe fluid resistance is not as large as the small pipe diameter, easy to clean, more used for viscous or dirty fluid.

 

WLD Company can provide 304/316 stainless steel heat exchange tubes from 10mm to 114mm, the thickness of 0.6mm to 3.0mm; The length can be customized according to your actual working conditions. If you need it please contact us today.

The stainless steel offers good corrosion resistance against most common chemical corrodents and industrial atmospheres. The stainless square or rectangle tubes have the advantages of long service life, good corrosion resistance and lightweight can be used in industrial piping, automotive, instrumentation, medical and construction industries, such as stair handrails, railings, partitions, bicycles, medical equipment, cars and so on. Here is the weight chart of 304 square and rectangle tubing:

304 Stainless steel square and rectangle tubing weight 

Length:6000mm, Unit:KG

サイズ	0.4	0.5	0.6	0.7	0.8	0.9	1	1.2	1.5	2	2.5	3	4	5
10×10	0.74	0.91	1.09	1.26	1.43	1.59
12×12	0.89	1.1	1.32	1.53	1.73	1.93	2.13	2.53
15×15	1.12	1.39	1.66	1.92	2.19	2.45	2.71	3.21	3.95
18×18	1.35	1.68	2	2.32	2.64	2.96	3.28	3.9	4.8
19×19	1.42	1.77	2.12	2.46	2.8	3.13	3.47	4.12	5.09	6.63
20×20	1.5	1.87	2.23	2.59	2.95	3.3	3.66	4.35	5.37	7.01
22×22		2.06	2.46	2.86	3.25	3.65	4.04	4.81	5.94	7.78
23×11		1.58	1.89	2.19	2.49	2.79	3.09	3.67	4.52	5.87
23×23		2.15	2.57	2.99	3.14	3.82	4.23	5.04	6.23	8.16
24×12		1.77	2.12	2.46	2.8	3.13	3.47	4.12	5.09	6.63
24×24		2.25	2.69	3.12	3.56	3.99	4.42	5.27	6.51	8.54
25×25		2.34	2.8	3.26	3.71	4.16	4.61	5.49	6.8	8.92
28×28		2.63	3.14	3.66	4.17	4.67	5.18	6.18	7.66	10.06
30×30		2.82	3.37	3.92	4.47	5.02	5.56	6.64	8.23	10.82
36×23		2.77	2.31	3.86	4.4	4.93	5.46	6.52	8.08	10.63
36×36		3.39	4.06	4.72	5.38	6.04	6.7	8.01	9.94	13.1
38×38				4.99	5.69	6.39	7.08	8.46	10.51	13.86
40×40				5.26	5.99	6.73	7.46	8.92	11.08	14.63
48×23			4	4.66	5.31	5.96	6.61	7.89	9.8	12.91
48×48				6.32	7.21	8.1	8.98	10.75	13.37	17.67
50×50				6.59	7.52	8.44	9.37	11.2	13.94	18.43	22.85
20×10	1.12	1.39	1.66	1.92	2.19	2.45	2.71	3.21
25×13	1.42	1.77	2.12	2.46	2.8	3.13	3.47	4.12	5.09	6.63
30×15		2.1	2.52	2.92	3.33	3.73	4.13	4.92	6.09	7.97
38×25			3.54	4.12	4.7	5.27	5.84	6.98	8.66	11.39
40×10			2.8	3.26	3.71	4.16	4.61	5.49	6.8	8.92
40×20			3.37	3.92	4.47	5.02	5.56	6.64	8.23	10.82
50×25			4.23	4.92	5.61	6.3	6.99	8.35	10.37	13.67
60×30				5.92	6.76	7.59	8.41	10.06	12.51	16.53	20.47
75×45				7.92	9.04	10.16	11.27	13.49	16.79	22.24
55×13			3.83	4.46	5.08	5.7	6.32	7.55	9.37	12.34
60×40				6.59	7.52	8.44	9.37	11.2	13.94	18.43	22.85
60×60				7.92	9.04	10.16	11.27	13.49	16.79	22.24	27.61	32.91
70×30				6.59	7.52	8.44	9.37	11.2	13.94	18.43	22.85
73×43				7.65	8.73	9.81	10.89	13.03	16.22	21.48	26.66
80×40						10.16	11.27	13.49	16.79	22.24	27.61	32.91
80×60						11.87	13.17	15.77	19.64	26.04	32.37	38.62	50.89
80×80						13.58	15.07	18.05	22.5	29.85	37.13	44.33	58.5
95×45						11.87	13.17	15.77	19.64	26.04	32.37	38.62	50.89
100×40							13.17	15.77	19.64	26.04	32.37	38.62	50.89
100×50							14.12	16.91	21.07	27.95	34.75	41.47	54.7
120×60								20.34	25.35	33.66	41.88	50.04	66.12	81.9
150×100									35.34	46.98	58.53	70.02	92.76	115.2
100×100								22.62	28.21	37.46	46.64	55.74	73.73	91.41
150×150									42.48	56.52	70.43	84.29	111.79	138.99