ttl for secondary zones is set to some default values.

while creating secondary zone . ttl is set to 3600 .

vyom@vyom:~$ openstack zone create temp.org. --type SECONDARY --masters vyom.org.

+----------------+--------------------------------------+
| Field | Value |
+----------------+--------------------------------------+
| action | CREATE |
| created_at | 2015-10-20T09:51:07.000000 |
| description | None |
| email | <email address hidden> |
| id | 059572a2-b963-484f-8b72-e18d8fa9774e |
| masters | vyom.org. |
| name | temp.org. |
| pool_id | 794ccc2c-d751-44fe-b57f-8894c9f5c842 |
| project_id | 237404bcb8a34bc88f1a401c7439654b |
| serial | 1 |
| status | PENDING |
| transferred_at | None |
| ttl | 3600 |
| type | SECONDARY |
| updated_at | None |
| version | 1 |
+----------------+--------------------------------------+

which according to http://docs.openstack.org/developer/designate/rest/v2/zones.html
should be NULL for SECONDARY zones.