Problem Solving and Corrosion of Cast Iron. Presented by: Ir Dr George Greene SAFE and HKU

Transcription

1 Problem Solving and Corrosion of Cast Iron Presented by: Ir Dr George Greene SAFE and HKU

2 Outline of the presentation Early stages of problem solving Some corrosion basics Graphitic corrosion of cast iron Internal corrosion environment External corrosion environment Examples with cause and effect charts

3 This presentation will introduce two early stages of problem solving and then consider the corrosion of cast iron but two of the examples will be illustrated by cause and effect analysis charts that help to understand the reality of how and why the problems occurred. The charts help others to understand the reasoning and outcome of an investigation and can form an part of a report. The charts also helps to identify possible solutions to the problems.

4 Problem Solving Problem Definition Cause and Effect Analysis Identify possible solutions Evaluate possible solutions Implement and follow up selected solutions

5 Problem definition What? When? Date, time, relative to other things? Where? Town, district, plant, equipment, location Significance In dollar terms if possible Health, Safety, Environment, statutory reporting, repair cost, subsequent damage costs, lost production, medical costs, public relations cost and frequency

6 Do not ask who. This is premature and may stop people from cooperating with efforts to solve a problem. Do not ask how or why. These are the objectives of the efforts is to find the causes of how and why something happened. There may be more than one statement of what the problem is based on different peoples view point and responsibilities.

7 Cause and Effect Charting Focus on the problem and forget about solutions. You need to understand the problem first. Start with the what of the problem definition as the primary effect and work backwards in time. Ask what were the direct and necessary causes of that primary effect?

8 Repeat the question for each of the causes that are actions and conditions Time

9 Simple statement of a problem: A bottle is dropped and the contents spilt when the bottle breaks. A simple answer is to clean up the spill. But if there are frequent spills or the consequences are serious then finding a lasting solution might be the best answer. Can you find an effective solution immediately?

10 A more thorough approach might be justified depending on the seriousness of the incident. The problem definition what is the spilt contents, the most recent effect. When and where should be straight forward. But what of the significance?

11 Liquid: water, milk, red wine, oil, ammonia, cyanide or mercury Where: kitchen, onto an antique carpet, on some stairs, beside a swimming pool, in an airport lounge or in an airplane. The significance and urgency would vary considerably due to the health and safety issues as well as the cost of damage and disruption.

12 What causes can you think of that might be significant? You call them out and I will make a list.

13 Primary Effect is Liquid spilt Actions and conditions could be: Bottle hit floor Floor hard Sufficient force Bottle cold Condensation on Glass bottle Bottle slippery Grip on bottle Glass brittle Bottle hit floor Bottle broke Carrier very young Bottle filled Bottle existed Gravity Bottle fell

14 This different way of thinking helps you or a team of people to focus on the problem and justify time spent on understanding and finding solutions. It develops a logical and evidence based understanding of how the problem arose and can form the basis for identifying solutions.

15 Some corrosion basics Corrosion is a natural process and all metals react with their environment to some extent. During corrosion the chemical potential energy decreases just as mechanical potential energy decreases when a ball is dropped or water runs downhill.

16 Electrochemical corrosion always requires: Anode Cathode For the oxidation reaction and loss of metal For the reduction reaction Electrolyte Liquid for ion transport Conducting path For electron transport

17 Simple electrochemical corrosion cell Conducting Path Anode Cathode Electrolyte

18 Ideally corrosion is uniform over the surface. This is assumed in the galvanic series and by design engineers when they provide a corrosion allowance. Unfortunately corrosion is normally not uniform but is uneven or possibly highly localised so the effect is much more significant than the average weight loss of uniform corrosion would be.

19 Uneven corrosion can be seen in a steel pipe where mounds of corrosion product form over some areas.

20 The first thing to do when meeting a corrosion problem is to identify the four requirements of electrochemical corrosion. In particular, why has more corrosion occurred in one location than in another. The area of attack is the anode and areas of little or no attack are probably the cathode. In most cases the electrolyte and conducting path will be clearly evident but they should be noted.

21 After corrosion starts either at random or under some debris or silt, the corrosion product that has formed shields the corroding area. Oxygen cannot access the corroding area as easily as away from it. A difference in oxygen concentration occurs and if all other factors are the same corrosion will continue at the anode and the rest of the surface will act as the cathode.

22 Schematic diagram of the cross section of a water drop to show how under deposit corrosion occurs due to difference in oxygen concentration. Supply of oxygen is easiest near the edge of the drop.

23 The corrosion occurs where the oxygen concentration is lower. I know that does not sound right at first but it is true if the only difference in the environment is in oxygen concentration. The high oxygen at the cathode results in consumption of electrons and that provides the driving force for the reaction and the attack at the anode that releases electrons to the cathode.

24 With gray cast iron the corrosion continues and takes the form of graphitic corrosion where some of the iron is removed to the electrolyte or forms a loose corrosion product over the anodic area. The rest of the iron in that area is oxidised leaving a weak mixture of iron oxide and the original graphite flakes. The same thing can happen with ductile iron but not normally as rapidly.

25 Micrographs of ductile cast iron and the dense corrosion product showing graphite Cast iron Dense corrosion product Iron Graphite spheriods Dense iron oxide corrosion product 25

26 Iron has been removed from the outer surface of this cast iron pipe. The leaching of iron has extended almost halfway through the thickness. There is almost no strength in the affected material that is just rust and graphite.

27 Another example of the removal of iron from a cast iron component. The component apparently retains the original dimensions but the remaining cast iron is only about 25% of the original thickness. Cast iron sea water pipes are affected in this way and some have failed suddenly in Hong Kong.

28 Remaining wall thickness Original wall thickness

29 Location where the graphitic corrosion had fully penetrated the wall thickness and resulted in leaking. No cast iron remained. When the pipe was sectioned the corrosion product broke off.

30 Longitudinal section Microscopic photos of the cast iron and the dense corrosion product 30

31 The dense corrosion product had broken off along the crack in this view of the inside of the pipe revealing the metal. 31

32 Some of the iron is lost into the electrolyte or forms a loose corrosion product on the surface and the cast iron may appear to retained the original thickness. If scraped lightly with a screw driver the surface may be shiny and appear to be metal. However, that shiny appearance is just the graphite in the corrosion product smeared over the surface. This can be misleading during site inspection. You can use the screw driver to dig out the weak corrosion product until you get to solid metal.

33 Very localised forms of corrosion are normally not a problem with cast iron but can be for other alloys including stainless steels which is not really stainless. Remember STAINLESS STEEL IS NOT STAINLESS

34 Corrosion cast iron internal Commonly Environment fresh, salt, or stagnant water or other conditions including sewage, waste water or chemical discharge Protective coatings bitumen, epoxy, cement

35 Example leakage from a strainer body A hole developed through the cast iron body of a sea water strainer with 6 months of commissioning. The owner wanted to know why. Would it happen again? What could be done to control the problem? Visual inspection on site followed.

36 Weld repairs had already been made on the strainer. No observation could be made of the hole original details and supplied photos were not detailed enough to assist.

37 In one area the coating came away from the surface very easily when the mud and slime film was wiped off.

38 The area under the failed coating was clean steel with some local pitting marks. The cleanliness of the surface indicated active corrosion had been in progress.

39 There were also some isolated spots of corrosion where pitting could have taken place like at the leakage point.

40 Another area was corroded with some indicated pitting. Picture taken before cleaning.

41 Again when washed clean, the coating in this area was stripped off leaving a very clean metal surface with some pitting

42 The strainer was fully coated internally with a black material said to be epoxy. Ultrasoninc tests indicated variable thickness. Welds repairs had not been ground down.

43 Clearly the coating work was not adhering to the surface correctly. This and other strainers were recoated on site. After one month normal service the strainer was opened again and a site visit made for inspection. There were numerous small spots of corrosion on the surface. This early inspection allowed localised repairs to be made at defects without the need to completely recoat the strainer. It also showed that cathodic protection was necessary.

44 Assessment of the causes of the localised attack Sea water was clearly the electrolyte and all part were connected providing a conducting path. Why did the attack occur at fairly random points? There must have been some weak points in the coating, holidays or damage to the coating. These coating defects were no longer visible. The direct evidence was lost during the corrosion. The underlying cast iron was exposed to the sea water so corrosion could start at those anodes.

45 Why had the attack been so concentrated and rapid? Only a small number of anodes were formed at the defects in the coating. A large area of the stainless steel basket acted as the cathode to consume electrons. The result was a corrosion current with electrons supplied to the large cathode by small anodes so the current density at the anodes was very large.

46 Recommendations: 1. Follow the design specifications when applying the coating. 2. Dress the repair welds to allow a good coating 3. Make two applications of the coating to reduce the probability of defects and holiday test. 4. Inspect soon after commissioning to locate, clean and recoat areas of local attack at an early stage. 5. Install zinc anodes to provide cathodic protection to the small areas of exposed cast iron. Most important

47 Distribute a draft cause and effect reality chart

48 Failed main sea water supply pipe Who Insurance brokers Request Why did the pipe fail? 48

49 This was a serious case with more information supplied and a number of complicating factors that are not included here. This presents the examination of the failed cement lined, ductile cast iron pipe. 49

50 Two pieces from the pipe The location of the fracture 50

51 Another piece of the pipe Location of the fracture 51

52 View of the end of the failed pipe that remained in the threaded flange assembly 52

53 Detailed view of the retained end of the pipe in the threaded flange Flange threads 53

54 Filed edge of length of failed pipe away from the fracture. Metal is shiny, the corroded layer is dark and the cement lining is light. 54

55 Measurements around the circumference of the pipe at a position away from the fracture 55

56 Measurements around the circumference of the pipe at a position near the fracture 56

57 Cut off of the end with the fracture surface Development view with three photos around the circumference of the pipe 57

58 The very end of the fractured pipe, the threaded end, remained in the flange. An attempt to cut a section with a hacksaw found that the saw cut through very easily. A second cut was made and a section of the threaded pipe lifted clear of the flange thread. 58

59 A 59

60 Two pieces of the pipe thread were very light. These are pieces of the dense corrosion product that is mainly iron oxide and the graphite from the cast iron. Most of the iron has been dissolved. 60

61 Sketch of the assessed corrosion damage to the flange joint 61

62 Micrographs of the cast iron flange and the dense corrosion product Cast iron flange Dense corrosion product Iron Graphite spheriods Dense corrosion product 62

63 The weakness of the threaded flange joint was an obvious and main factor in the pipe failure although some additional factors may have precipitated the actual failure at that particular time. The real questions were about the condition of other threaded joints. It this typical? Can they be examined by NDT? What precautions can be taken? 63

64 When the pipe was cut to length and a thread was cut, it was difficult to protect the cut end of the pipe from corrosion. Over 30 years the corrosion gradually weakened the joint. It is probable that a pressure surge in the system triggered the failure but the primary cause was the weakened condition of the pipe.

65 It is possible to conduct ultra sonic inspection to assess the thickness of pipes but this is only a sampling and may not be a good indication of uneven thinning. It is also very difficult to inspect joints unless they are in a very advanced state of deterioration.

66 A water box of a heat exchanger developed leaks after about three years

67 General view of the return water box. The entire internal surface was coated and zinc anodes were used for cathodic protection. 67

68 Close up of the top section Holes numbered 2, 3, 5 and 9 with corrosion damage (a total of 14 of 72 holes were affected by the corrosion) 68

69 The corrosion attack removed enough metal from the inside of the flange so that water leaked out to the bolt holes in into the plant room. Water A. As designed B. As observed where corrosion occurred 69

70 Zinc anodes had corroded to a large extent when compared to new anodes. The used anode had been cleaned but the thickness of the removed corrosion product was unknown and that layer may have reduced the anode effectiveness. 70

71 Close up of holes 1 to 5 showing attack at holes 2,3 and 5 but not 1 and 4. 71

72 Three main questions arose: Why did corrosion occur at 14 holes but not at the other 58 holes? Why only on the flange and not on the cover? Why did the anodes not protect the cast iron? 72

73

74 Close up of holes 1 to 3 74

75 Close up of holes 2 to 5 75

76 There appeared to be deformation at the edge of holes 1 and 4 where corrosion did not occur. It seemed highly unlikely that the cast iron could be deformed by the rubber gasket. Could the coating have been deformed? Could the gasket placement have varied at different holes? 76

77 Close up of holes 1 to 5 on the end plate showing the imprint of the gasket. 77

78 Gasket held in position over a hole that had not been attacked by the corrosion 78

79 Gasket held in position over a hole that had been attacked by the corrosion 79

80

81 Explanation of how it probably happened: The coating on the flange could have been soft and been squeezed out from under the gasket toward the edge to form the apparent deformed regions. Alternatively the coating may have been more rigid but in both cases the coating could be cracked and/or damaged where the gasket overlapped the edge of the flange. 81

82 If the combination of the gasket extending over the edge of the flange (C) and excess coating at edge of the flange both occurred at the same bolt hole, then damage could occur to the coating when the joint was tightened.

83 Cathodic protection from the zinc may have become less effective as corrosion product built up on the anode and slowed the dissolution of the anode. The supply of electrons from the zinc would decrease and be unable to protect the small exposed areas of cast iron where the coating had been damaged. 83

84 The investigation did not continue as a preliminary report was sufficient for the client s requirements. Better maintenance of the anodes might have prevented the corrosion but the primary factor was the damage to the coating. Several aspects of manufacturing and assembly could be addressed to prevent damage to the coating and formation of localised anodic areas on the cast iron. 84

85 The corrosion damage to the cast iron water box could be repaired with fibre reinforced epoxy so that the geometry of the flange could be restored. The actual damage was limited but it would have become more serious with time. 85

86 I hope this has been of interest and you will look over the supplied cause and effect reality chart to see how effective they can be. Any questions?

87 Buried Not accessible General soil environment clay alkaline, acid, salt, reclaimed land, sewage contamination that can vary along the pipe and around the circumference. Trench preparation, bedding, backfill, debris also determine the environment Protection bitumen, epoxy, tape + cathodic protection (sacrificial or impressed current)

88 Nearby leaking sewer pipe will alter the environment and may result in microbiologically enhanced corrosion. Sulphate reducing bacteria function in anaerobic conditions often resulting in black corrosion product and the smell of hydrogen sulphide. Laboratory analysis of soil samples is necessary to confirm this specific form of corrosion.

89 A hole in a pipe was found that was large and unusual. The was a sharp edge on one side and a rounded edge on the other side of the hole. There was very little corrosion on the rest to the pipe. It was learned that there was a different, sea water, supply pipe very close by and it had a small hole in it facing the subject pipe.

90 Only brief examination of the pipe was permitted by the client but the following was concluded. The large hole had been formed by an oblique stream of seawater from the other supply pipe that had picked up sand and suspended solids from the soil and blasted the exterior of the pipe until the large and unusual hole had formed.

91 Corrosion of cast iron pipes external Above ground Environment Humidity, salt spray or condensation Accessibilty For inspection and maintenance can vary considerably Protective coatings paint, bitumen, epoxy, or inhibited tape

92 Buried pipe cracked along the bottom after many years service Illustrations from the recent pipe failure

93 Lining of pipes has been done in a number of cases in Hong Kong with noted success. There remain some questions about the effect at branches in the pipe where the lining is cut away to allow water to flow to the branch.

94 I hope this has been of interest and you will look over the supplied cause and effect reality chart to see how effective they can be. Any questions?