AIRCRAFT MAINTENANCE MANUAL - Job Instruction Cards

CHEMICAL PASSIVATION PROCESS FOR STAINLESS STEEL ALLOYS

1. General

A. Reason

This topic gives the general instructions for the passivation process for
corrosion-resistant steel alloys. This type of bath is applicable for
stainless steel and Inconel parts and for bleaching titanium alloy parts. A
passivation bath neutralizes an active surface undergoing corrosion and
transforms it into a practically inactive surface by forming a surface
layer of oxide.

2. Specifications

WARNING: SOME OF THE MATERIALS USED IN THIS PROCESS MAY BE TOXIC, FLAMMABLE OR
IRRITATING TO THE SKIN. THE USER IS SOLELY RESPONSIBLE FOR CONSULTING
THE APPROPRIATE HEALTH AND SAFETY AUTHORITIES FOR SPECIFIC
REQUIREMENTS RELATED TO FACILITIES, EQUIPMENT, VENTILATION AND
HANDLING PRECAUTIONS.

A. Products

- Nitric acid, technical 40-42 Bè.

- Fluoridric acid 70%.

- Dichromate Sodium (2H20).

- Turco 4338 (D8908, F3172, 61102 (Ref. 20-31-00)).

- Surface activator:
. Activol 1357 (D2426 (Ref. 20-31-00)) or
. Turcoform Ti-Etch Additive 200 (D8908, F3178, 61102 (Ref. 20-31-00)).

B. Equipment

WARNING: MEANS OF PROTECTION: ACID-RESISTANT CLOTHING AND GLOVES, MASK,

GOGGLES.

- Pumice powder, FF grade or, as on alternative, Tri poli powder.

- Scotch Brite, very fine grade or extra fine, 3 Minnesota.

- Aluminium oxide abrasive paper, grit of various sizes.

- Wire brushes, stainless steel.

- White cotton gloves.

- Neutral Kraft paper.

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- Polyethylene film.

- Tank and accessories: steel jig, PVC tank, PVDF coating, tools.

- Stirring: bath stirred by filtered and de-oiled compressed air.

- Ventilation: steam eliminated by suction and rinsed off by water.

- Automatic or manual titrator.

- Combined glass electrode or pH reference electrode / combined Pt

electrode pair.

- 1 ml pipette.

3. Procedure

A. Cleaning, decontamination and passivation, Type A (see Fig. 1)

NOTE: This method is not applicable for decontamination of lead steels.

(1) Solvent vapor clean (Ref. 20-24-05) or manual clean (Ref. 20-24-06).

(2) Degrease and rinse.

(3) Remove oxide, if any.

(4) Passivate by immersion for 20 minutes in solution No. 1 or No. 2 (see

Table 1) unless otherwise specified in the following notes:

NOTES: 1. Solution No. 1 shall not be used to passivate PH series

steels (precipitation-hardenable) and series 400
(martensitic) for DACO parts.

2. Solution No. 2 shall not be used to passivate BCAC parts.

(5) Rinse in cold water.

(6) Process Class 2 parts by immersing them for 30 minutes in solution

No. 3 (see Table 1).

(7) Rinse in cold water.

(8) Rinse in water, preferably hot (65°C (149°F)) and dry with hot or cold
filtered air.

(9) Embrittlement relief of corrosion resistant steels passivated in

solution No. 1 or 2 (see Table 1).

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B. Cleaning and deoxidation with nitrofluoridric acid Type B (see Fig. 2)

(1) Solvent vapor clean (Ref. 20-24-05) or manual clean (Ref. 20-24-06).

(2) Degrease and rinse.

(3) Condition oxides, if required, by immersion in solution No. 5 (see

Table 1) for 15-60 minutes max.

(4) Rinse in cold water.

(5) Pickle by immersion in nitrofluoridric acid, solution No. 4 (see

Table 1) for 5 minutes max.

NOTE: Operations B. (3) through B. (6) may be repeated several times

if necessary. Nitrofluoridric acid immersion cumulative time
shall never exceed 30 minutes.

(6) Rinse in cold water.

(7) Rinse using water preferably hot (65°C (149°F)) and dry with hot or
cold filtered air.

(8) Embrittlement relief is not required for series 300 steels to be

treated using this method.

C. Mechanical deoxidation – Type C (Ref. Fig. 3)

(1) Solvent vapor clean (Ref. 20-24-05) or manual clean (Ref. 20-24-06).

(2) Method 1 – Dry sandblasting

(a) Mask with adhesive tape, rubber, etc. areas not to be

sandblasted.

(b) Blast surfaces with aluminum oxide sand grit 100-180 mesh or
finer.

NOTE: Sand used for stainless steel shall not be used for other
metals.

(c) Eliminate sand residues using filtered compressed air or by

rinsing and drying by air (if required).

(3) Method 2 – Brushing

(a) Brush affected surfaces using series 300 steel wire brushes
manual or powered.

(b) Remove residual products (Ref. Para. C. (1)) by alkaline and/or

degreasing and rinsing.

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(4) Method 3 – Sandpapering/Pumicing

Sandpaper and/or pumice affected areas using cotton or scotch brite

pads containing products specified in Para. 2. until oxides and/or
contamination are uniformly removed.

Manually remove residues and/or alkaline degrease and rinse.

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Solvent cleaning

Alkaline cleaning and rinsing

Oxide removal

Passivation

Rinsing

Treatment (class 2 only)

Rinsing

Hot water rinsing and drying

Embrittlement relief, if required

Cleaning, Decontamination
and Passivation
Figure 1
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Solvent cleaning

Alkaline cleaning and rinsing

Parts without oxides or

Parts with heat
with light oxides and
treatment oxides
discolorations

Oxide conditioning

Rinsing

Nitrofluoridric pickling

Rinsing

Do not dry parts

Hot water rinsing and
drying

Embrittlement relief,
if required

Cleaning and Deoxidation

with Nitrofluoridric Acid
Figure 2
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Solvent cleaning

Method 1 Method 2 Method 3

Dry Sandblasting Brushing Sandpapering/Pumicing

Masking Brushing Sandpaper and/or

Pumice surfaces

Sandblasting Removal of Residue removal

brushing residues

Dust removal

Complete finishing
within 4 hours

Mechanical Deoxidation
Figure 3
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 AIRCRAFT MAINTENANCE MANUAL - Job Instruction Cards

SOLUTION COMPONENTS X EACH LITER WORKING PARAMETERS

OF THE BATH
CONCENTRATION TEMPERATURE
g/l (O/g) °C (°F)
No. 1 Water Balance 280-310
Inhibited to 1 lt. (37.52-41.54)
57-63
Nitric Acid Nitric Acid 200 cc
(134.6-145.4)
Sodium Dichromate 23 gr 21-25
(2.81-3.35)
No. 2 Water Balance
655-735 49-54
Nitric Acid to 1 lt.
(87.77-98.49) (120.2-129.2)
Nitric Acid 480 cc
No. 3 Water Balance
50-65 60-70
Sodium To 1 lt.
(6.7-8.71) (140-158)
Dichromate Sodium Dichromate 60 gr
No. 4 Water Balance
To 1 lt.
Nitro Fluoridric Acid 30 cc 24-60
Fluoridric (3.22-8.04)
Nitric Acid 450 cc 560-670
(75.04-89.78)
Surface active: 19-35
Activol 1357 or 0.2 cc (66.2-95)
Turcoform Etch 15 mg

Dissolved Metals:
Iron - - - 5 (0.67) max.

Titanium - - - 10 (1.34) max.

No. 5 Water Balance
Oxide to 1 lt.
Conditioner Turco 4338: 260 gr
88 (190.4)
up to
Alkalinity - - - 238-299
boiling point
(31.89-40.07)
Potassium - - - 47-59
permanganate (6.3-7.91)

Solutions and Water

Table 1
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4. Controls

A. Process control

Inspection shall verify that all process parameters have been met.

B. Baths concentration

Chemical Laboratory shall check chemical solutions prescribing adjustment

required to maintain them within Table 1 chemical-physical limits.

For the surface tension test deviation from Table 1 values is tolerated.

C. Rinsing water pollution

Chemical Laboratory shall perform a check, at a frequency established

according to facility characteristics and to required amount of production.

D. Visual check of processed parts

Parts shall be examined for defects such as: localized, irregular or

excessive etching, surface contamination, oxide residues, etc.

When doubt arises, all processed parts shall be checked for sizes and
intergranular etching which shall always be within the limits (0.0002 in.
max. for intergranular etching and 0.001 in. max. for endgrain pitting).

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