1、Minimizing er of Alum FOURTH EDITION, JANUARY 2004 I www.aluminum.org The 2004, fourth edition has been revised through the support of the Technology Committee of the Sheet and Plate Division, ALUMINUM ASSOCIATION SHEET AND PLATE DIVISION COMPANIES Alcan Inc. Alcoa Inc. AMAG Rolling GmbH ARCO Alumin
2、um, Inc. Coastal Aluminum Rolling Corus Aluminium Rolled Mills, Inc. Products, USA Jupiter Aluminum Corporation Kaiser Aluminum and Chemical Corporation Logan Aluminum, Inc. Nichols Aluminum Ormet Corporation Pechiney Rolled Products Gulf Aluminium Rolling Mill United Aluminum Co. Corporation About
3、The Aluminum Association The Aluminum Association, based in Washington, DC with offices in Detroit, MI, represents US. and foreign- based primary producers of aluminum, aluminum recyclers and producers of semi-fabricated products as well as suppliers to the industry. Member companies operate almost
4、200 plants in North America and many conduct business worldwide. NOTICE Disclaimer The use of any information contained herein by any member or nonmember of The Aluminum Association is entirely voluntary The Aluminum Association has used its best efforts in compiling the information contained in thi
5、s book. While the Association believes that its compilation procedures are reliable, it does not warrant, either expressly or implied, the accuracy or completeness of this information. The Aluminum Association assumes no responsibility or liability for the use of the information herein. All Aluminum
6、 Association published standards, data, specifications and other technical materials are reviewed and revised, reaffirmed or withdrawn. Users are advised to contact The Aluminum Association to ascertain whether the information in this publication has been superseded in the interim between publicatio
7、n and proposed use. The Incorporated Aluminum Association 900 19th Street, N.W., Washington, D.C. 20006 Guidelines for Minimizing Water Staining of Aluminum Contents 1 . Introduction . 4 2 . Causes of Water Stain 4 A.ExternalSources . 4 B . Moisture Condensation 4 3 . Packaging . 7 4 . Inspection Pr
8、ocedures 7 5 . Prevention . 8 6 . Conclusion 9 Appendices 1 . Aluminum Water Stain Prevention . 10 2.A. Measurement Instrument Suppliers . 1 0 2.B. Suppliers of Moisture indicating Labels . 10 2.C. Suppliers of Water Stain Preventatives 10 3 1. Introduction but may cause processing problems where ad
9、ditional surface finishing or fabrication is to be performed. Virtually ail staining problems occur during shipping, handling, or storage. Some of the incorrect practices that result in water staining and ways to minimize or avoid the staining are discussed in this pamphlet. A summary of the measure
10、s to minimize water staining is provided in Appendix 1 and is reproduced as a handy wall chart, which is included with this pamphlet. Copies of the chart may be obtained separately from The Aluminum Association. Aluminum is naturally attractive and resistant to corrosion. Unlike most other metals, i
11、t takes care of itself. When exposed to dry or moist air it combines with oxygen to form a tough, transparent, protective oxide coating. In the presence of moisture, aluminum will not rust as will steel, but under certain conditions aluminum will stain. Such stains are usually found objectionable fo
12、r esthetic reasons; they have no significant effect on strength 2. Causes of Water Stain Aluminum and other metals can stain when water is trapped between wraps, sheets or other mating surfaces. If there is no air flow to remove the water, prolonged contact between the water and the metal causes a r
13、eaction between the two which results in water stain. Figures 1 and 2 show examples of water staining on aluminum coiled and flat sheet, respectively. There are two main sources of this water: A. External Sources Obvious sources are rain, snow and water leaks. Water coming in contact with the surfac
14、e of the aluminum from sources such as a leaking tarp on a truck or a leaking roof in a storage area has the potential for causing water stain. B. Moisture Condensation This is the more complex of the two sources and therefore harder to understand and control. Air normally contains moisture in the f
15、orm of invisible water vapor. Relative humidity, expressed as a percentage, provides a measure of the amount of water vapor actually in the air compared to the total amount of water vapor that the air can hold. As the temperature of air increases, its capacity to hold water vapor increases. Dew poin
16、t is the temperature at which water vapor from the air begins to condense and is affected by the relative humidity and temperature of the air. The dew point can be determined from a chart (see Table i), based on simple measurements of air temperature and relative humidity. The type of instrument use
17、d to measure (and log) temperature and relative humidity is shown in Figure 3. The older style of thermometers containing mercury should not be used because mercury from broken thermometers can corrode aluminum and other metals. Some suppliers of these instruments are listed in Appendix 2.A. Figure
18、1 - Water Stain on Coil Figure 2 - Water Stain on Sheet 4 Figure 3 - Digital Instrument for Measuring Temperature this sets up the condition where water begins to condense on the surface of the aluminum. and spring months but may occur whenever there are large enough fluctuations in temperature and
19、humidity. Obviously this process is not limited to fall Figure 4 - Condensation on Cold Aluminum Cans During Loading: Loading metal removed from a cool or cold storage area into a warm trailer or railroad car, on a humid day, can result in condensation on the aluminum. This can occur during any seas
20、on of the year; however, for some areas it may be more pronounced in summer. During Unloading-Moving Cold Metal Into Warm Storage: Condensation at the unloading point is more likely to occur during the cooler months. The following example may help to clarify this point: Metal at 16C (60F) in a wareh
21、ouse is loaded into a trailer and shipped. The metal is in transit for about two days. The outside temperature is - 1C (30F). Within a two-day transit period, the temperature of the metal gradually decreases to -1C (30F). When the aluminum reaches its destination, it is unloaded and moved directly i
22、nto a warehouse where the temperature is 16C (60F) and the relative humidity is 50%. From Table 1, the dew point of the air is 5C (41F). Since the temperature of the aluminum is now lower than the dew point of the air, conditions are ideal for water to condense on the surfaces such as the edges of s
23、tacked sheet or coils. The condensed moisture can enter between the sheets and wraps by capillary action and may produce water stain. 5 Table 1 : Dew Point Calculator Air Temp “C 43 Air TemDerature in Dearees Celsius - Yo Relative Humidity 100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 43
24、 42 41 40 39 38 37 35 34 32 31 29 27 24 22 18 16 11 5 35 32 29 41 I 41 39 38 37 36 35 34 33 32 29 28 27 24 22 19 17 13 8 3 - 35 34 33 32 31 30 29 27 26 24 23 21 19 17 15 12 9 4 O 32 31 31 29 28 27 26 24 23 22 20 18 17 15 12 9 6 2 O 29 28 27 27 26 24 23 22 21 19 18 16 14 12 10 7 3 O 38 I 38 37 36 35
25、34 33 32 30 29 27 26 24 22 19 17 14 11 7 O 24 21 18 - 24 23 22 21 20 19 18 17 16 14 13 11 9 7 5 2 O 21 20 19 18 17 16 15 14 13 12 10 8 7 4 3 O 18 17 17 16 15 14 13 12 10 9 7 6 4 2 O 27 I 27 26 25 24 23 22 21 19 18 17 15 13 12 10 7 4 2 O 13 10 7 13121110 9 8 7 64 3 2 1 O 10 9 8 7 7 6 4 3 2 1 O 766443
26、210 161 16 14 14 13 12 11 10 9 7 6 5 3 2 O Air OF Temp Yo Relative Humidity 100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 41443210 110 105 100 95 21 2 1 o 110 108 106 104 102 100 98 95 93 90 87 84 80 76 72 65 60 51 41 105 103 101 99 97 95 93 91 88 85 83 80 76 72 67 62 55 47 37 100 99 97
27、95 93 91 89 86 84 81 78 75 71 67 63 58 52 44 32 95 93 92 90 88 86 84 81 79 76 73 70 67 63 59 54 48 40 32 01 o 90 85 80 75 70 90 88 87 85 83 81 79 76 74 71 68 65 62 59 54 49 43 36 32 85 83 81 80 78 76 74 72 69 67 64 61 58 54 50 45 38 32 80 78 77 75 73 71 69 67 65 62 59 56 53 50 45 40 35 32 75 73 72 7
28、0 68 66 64 62 60 58 55 52 49 45 41 36 32 70 68 67 65 63 61 59 57 55 53 50 47 44 40 37 32 65 60 55 50 65 63 62 60 59 57 55 53 50 48 45 42 40 36 32 60 58 57 55 53 52 50 48 45 43 41 38 35 32 55 53 52 50 49 47 45 43 40 38 36 33 32 50 48 46 45 44 42 40 38 36 34 32 35 32 45 45 43 42 40 39 37 35 33 32 40 I
29、 40 39 37 35 34 32 35 34 32 32 6 Example: Read the air temperature in the left hand column and the humidity at the top of the above chart. If the temperature of the storage area is 13C (55F) and the relative humidity is 6O%, the intersection of the two shows the dew point of the area to be 4C (40F).
30、 If the metal coming in is below 4C (40“F), water will condense on the metal. Packaging Packaged aluminum coils, flat sheet, and plate should be wrapped with various papers, plastic films, or laminates that provide reasonable protection against moisture penetration during shipment and storage. Coils
31、 should have caps on the ends for either vertical or horizontal stacking and shipment. When using a shrink wrap, it should envelop the entire package to ensure any condensation has an opportunity to run off and not penetrate the package. However, it should be recognized that any such packaging may n
32、ot be air- tight, and changes in temperature or humidity may still result in condensation on the aluminum surfaces. Packaged aluminum, even if protected with “waterproof materials, should not be stored outdoors. Wrapping materials should be left intact (and repaired if torn) until the aluminum is us
33、ed. If moisture is condensing on the packaging as shown in Figure 5, the moisture should be removed but not the packaging. If the packaging is soaked or if the aluminum is wet, then the procedures mentioned on page 9 should be followed. When surface quality is critical, aluminum flat sheets and plat
34、e should be additionally interleaved with special paper, foam sheets or other materials to protect the surfaces against abrasion. Even though some interleaving papers contain an additive to inhibit water staining, the normal precautions during transit, handling, and storage should be followed. It is
35、 very important that the coil or sheet metal package is securely attached to the skid to eliminate any possibility of movement which can tear or otherwise compromise integnty of the packaging during shipping. Use of desiccants is not recommended due to rapid overload conditions that can occur during
36、 long routes and the very large quantity of moisture that can accumulate on cold surfaces. Figure 5 - Moisture condensation on packaged metal inspection Procedures Because there is normally a time limit on carrier claims, incoming inspection becomes a vital part of the receiving process. Without inc
37、oming inspection the receiver cannot know what the condition of the metal is and how susceptible it may be to water staining. Without this knowledge the necessary steps to prevent the staining cannot be taken: A. If any physical or water damage is found on the skids, packaging or the metal, this sho
38、uld be noted on the receiving papers. Any package that is torn and exhibits a significant water accumulation should be considered suspect and scheduled for additional review and potentially targeted for immediate usage subject to production schedules and constraints. If possible the coil or flat she
39、et should be inspected and reoiled via recoiling or restacking operation. If it has been recorded, the temperature/ humidity data should be transmitted to the shipping point to assess particular conditions, track trends and develop improved routing, packaging or mode of transportation. B. Water vapo
40、r will condense on the surface of the metal if its temperature is lower than the dew point of the air in the storage area. (See Figure 6) It is important, therefore, to check the temperature of a few coils in every shipment and compare that temperature to the dew point of the storage area. (Refer to
41、 section 5. A.3) 7 The temperature of the aluminum can be easily measured by using a probe thermometer, as shown in Figure 7, or by cutting a 6 inch by 6 inch flap in the wrapping paper and placing a contact, dial-type thermometer on the surface of the aluminum, as shown in Figure 8. After recording
42、 the temperature, it is important to reseal the package by replacing and taping the flap. If not repaired, condensate could run down the inside of the wrap and result in water stain. C. An alternative method of identiing coils exposed to moisture is through the use of moisture tags (a.k.a. “Weeping
43、Eyes”). See Appendix 2.B. for a list of suppliers. These tags, made of moisture Figure 7 - Probe Thermometer Figure 8 - Contact thermometer absorbent material, can identi exposure to moisture by changes to the shape andor color of the tag depending upon the specific indicator used. Figure 9 shows a
44、new tag while Figure 10 shows the effect of exposure to a water mist. If the tag shows exposure to moisture, follow the directions in Appendix 1. Figure 9 - Unexposed Moisture Label Figure 1 O - Moisture Label after Water Mist Exposure 5. Prevention There are three methods of preventing water stain:
45、 A. Prevent water in any form from coming into contact B. If water is already in contact with the metal, remove it C. Consider precautionary measures to minimize water with the metal staining A. Prevent water in any form from coming To prevent the water from getting between wraps, sheets or other ma
46、ting surfaces, it is critical to prevent the temperature of the metal from getting below the dew point of the surrounding air. into contact with the metal 1. STORAGE: Intact packages should be stored in a dry area with good ventilation to ensure stabilization of the package. Temperature and humidity
47、 variations within the warehouse should be kept to a minimum by keeping outside doors closed. For surface critical products, air conditioned warehouses with limited access are preferred. If use of an air conditioned warehouse is not feasible, packages should be periodically inspected to veri that st
48、orage conditions have not deteriorated. Stock should be rotated as quickly as possible. It is also advisable to have forced circulation heaters in the storage area to maintain the temperature of the metal above the dew point of the ambient storage environment. Without fans to 8 distribute heat, stor
49、ing aluminum near walls, doors, windows, and other cooler areas may provide the conditions for condensation. should be stored outdoors unless water stain is not detrimental for the end use. Neither packaged nor bare aluminum 2. SHIPMENTS: For surface critical products it is important to minimize dayhight temperature cycling during typical storage and/or transit, and the preferred method is to use insulated trucks or rail cars. It is essential that long layovers at truck and rail depots are minimized. If this mode is not feasible due to metal shape or economic consid
