(8)After removing all corrosion visible through a 10-power magnifying
glass, apply corrosion treating solution, refer to par. 9.C.(4), and
wash thoroughly with clean water.
(9)Fair depressions resulting from rework per par. 7.C., and surface finish
with 400 or 600 grit abrasive paper. Select appropriate abrasive mate-rial per Fig. 3.
(10)Clean reworked area.
Volume 1
20-40-00
Page 13 OCT 01/01
(11)Determine depth of faired depression per par. 7.D., to ensure that depth
rework Limits have not been exceeded.
(12)Treat reworked area per 20-50-00.
C. Corrosion Removal -Chemical. Chromic acid solution may be used to remove
surface oxidation and light corrosion products from magnesium surfaces.
This method is not adequate where deep pitting or heavy corrosion have
occurred which require mechanical methods of removal If properly used, the
chemical method causes less reduction in section thickness. Parts contain-ing copper-based inserts shall not be treated with this method unless prop-erly masked off. Excessive amounts of fluorides, sulfates, or chlorides must
not be allowed to build up in the solution as these compounds tend to coat
and etch the metal rather than clean the surface.
CAUTION: THE FOLLOWING PROCEDURES ARE NOT APPLICABLE TO ADHESIVE BONDED PARTS
OR ASSEMBLIES, AREAS WHERE THE CHEMICAL AGENTS MIGHT BECOME LODGED.,
OR LOCAL AREAS BARED SPECIFICALLY FOR GROUNDING OR ELECTRICAL BONDING
PURPOSES.
(1)Positively identify the metal as magnesium.
(2)Remove loose corrosion products with aluminum wool.
(3)Mask off other materials and parts, especially rubber parts, bearings,
cast or pressed inserts, and plated steel to prevent contact with the
treating solution or its fumes.
(4)Prepare corrosion treating solution. Two chemical preparations are
available. To prepare the solutions, proceed as follows:
(a) Chromium trioxide preparation
WARNING: ADD CHROMIUM TRIOXIDE TO WATER; DO NOT ADD WATER TO THE CHROMIUM
TRIOXIDE.
1) To make 1 U.S. gallon of solution, add 24 ounces of chromium triox-ide to water in a mixing container of lead lined steel, stainless
steel or 1100 aluminum. Reaction time is 1 to 15 minutes when
heated to 190 to 202°F. Longer reaction time Is required when mixed
at room temperature.
(b) Sodium Dichromate/Nitric Acid Preparation. Prepare solution in the
following portions: 1-1/2 pounds of sodium dichromate, 1-1/2 U.S.
pints of concentrated nitric acid per U.S. gallon of water. Mix as
follows:
NOTE: The solution shall be prepared and stored in clean polyethylene or
glass containers.
1) Fill a suitable container with a volume of water equal to approxi-mately 1/4 the desired total quantity of solution.
2) Add full quantity of sodium dichromate in proportions indicated 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：757 防腐手册 CORROSION PREVENTION MANUAL 第二册 一般信息(27)
